Prevent the sidebar resizer to be over the preferences (codesandbox#245)

As reported in codesandbox#83.

Author: ValentinH

.all-contributorsrc

@@ -148,6 +148,15 @@
       "contributions": [
         "code"
       ]
+    },
+    {
+      "login": "ValentinH",
+      "name": "Valentin Hervieu",
+      "avatar_url": "https://avatars2.githubusercontent.com/u/2678610?v=4",
+      "profile": "http://valentin-hervieu.fr",
+      "contributions": [
+        "code"
+      ]
     }
   ]
 }

README.md

@@ -1,4 +1,4 @@
-# [CodeSandbox](https://codesandbox.io) [![Chat](https://img.shields.io/badge/chat-on%20discord-7289da.svg)](https://discord.gg/KE3TbEZ) [![All Contributors](https://img.shields.io/badge/all_contributors-14-orange.svg?style=flat-square)](#contributors) [![Build Status](https://travis-ci.org/CompuIves/codesandbox-client.svg?branch=master)](https://travis-ci.org/CompuIves/codesandbox-client)
+# [CodeSandbox](https://codesandbox.io) [![Chat](https://img.shields.io/badge/chat-on%20discord-7289da.svg)](https://discord.gg/KE3TbEZ) [![All Contributors](https://img.shields.io/badge/all_contributors-15-orange.svg?style=flat-square)](#contributors) [![Build Status](https://travis-ci.org/CompuIves/codesandbox-client.svg?branch=master)](https://travis-ci.org/CompuIves/codesandbox-client)
 
 An online code editor with a focus on React.
 
@@ -30,4 +30,5 @@ Thanks goes to these wonderful people ([emoji key](https://github.com/kentcdodds
 | [<img src="https://avatars0.githubusercontent.com/u/587016?v=3" width="100px;"/><br /><sub>Ives van Hoorne</sub>](http://ivesvh.com)<br />[💬](#question-CompuIves "Answering Questions") [📝](#blog-CompuIves "Blogposts") [🐛](https://github.com/CompuIves/codesandbox-client/issues?q=author%3ACompuIves "Bug reports") [💻](https://github.com/CompuIves/codesandbox-client/commits?author=CompuIves "Code") [🎨](#design-CompuIves "Design") [📖](https://github.com/CompuIves/codesandbox-client/commits?author=CompuIves "Documentation") [💡](#example-CompuIves "Examples") [🚇](#infra-CompuIves "Infrastructure (Hosting, Build-Tools, etc)") [👀](#review-CompuIves "Reviewed Pull Requests") [⚠️](https://github.com/CompuIves/codesandbox-client/commits?author=CompuIves "Tests") [🔧](#tool-CompuIves "Tools") | [<img src="https://avatars0.githubusercontent.com/u/887639?v=3" width="100px;"/><br /><sub>Donavon West</sub>](http://donavon.com)<br />[💻](https://github.com/CompuIves/codesandbox-client/commits?author=donavon "Code") | [<img src="https://avatars0.githubusercontent.com/u/5266810?v=3" width="100px;"/><br /><sub>Jeff Allen</sub>](http://www.jeffallen.io/)<br />[💻](https://github.com/CompuIves/codesandbox-client/commits?author=vueu "Code") | [<img src="https://avatars0.githubusercontent.com/u/1089897?v=3" width="100px;"/><br /><sub>Ben Gummer</sub>](https://github.com/bengummer)<br />[💻](https://github.com/CompuIves/codesandbox-client/commits?author=bengummer "Code") | [<img src="https://avatars3.githubusercontent.com/u/154732?v=3" width="100px;"/><br /><sub>James Gillmore</sub>](http://twitter.com/faceyspacey)<br />[💻](https://github.com/CompuIves/codesandbox-client/commits?author=faceyspacey "Code") [🐛](https://github.com/CompuIves/codesandbox-client/issues?q=author%3Afaceyspacey "Bug reports") | [<img src="https://avatars1.githubusercontent.com/u/9636410?v=4" width="100px;"/><br /><sub>Ade Viankakrisna Fadlil</sub>](https://musify.id)<br />[💻](https://github.com/CompuIves/codesandbox-client/commits?author=viankakrisna "Code") | [<img src="https://avatars1.githubusercontent.com/u/1854763?v=4" width="100px;"/><br /><sub>Tushar Sonawane</sub>](https://twitter.com/tushkiz)<br />[💬](#question-Tushkiz "Answering Questions") [💻](https://github.com/CompuIves/codesandbox-client/commits?author=Tushkiz "Code") [📖](https://github.com/CompuIves/codesandbox-client/commits?author=Tushkiz "Documentation") [🤔](#ideas-Tushkiz "Ideas, Planning, & Feedback") |
 | :---: | :---: | :---: | :---: | :---: | :---: | :---: |
 | [<img src="https://avatars3.githubusercontent.com/u/1239401?v=4" width="100px;"/><br /><sub>Johann Hubert Sonntagbauer</sub>](https://github.com/johann-sonntagbauer)<br />[🐛](https://github.com/CompuIves/codesandbox-client/issues?q=author%3Ajohann-sonntagbauer "Bug reports") [💻](https://github.com/CompuIves/codesandbox-client/commits?author=johann-sonntagbauer "Code") | [<img src="https://avatars2.githubusercontent.com/u/9586897?v=4" width="100px;"/><br /><sub>Joachim Seminck</sub>](https://github.com/jseminck)<br />[💻](https://github.com/CompuIves/codesandbox-client/commits?author=jseminck "Code") | [<img src="https://avatars3.githubusercontent.com/u/5210019?v=4" width="100px;"/><br /><sub>Subramanya Chakravarthy</sub>](http://chakrihacker.github.io)<br />[💻](https://github.com/CompuIves/codesandbox-client/commits?author=chakrihacker "Code") | [<img src="https://avatars3.githubusercontent.com/u/23088?v=4" width="100px;"/><br /><sub>Robert (Robby) O'Connor</sub>](http://robby.oconnor.ninja)<br />[🚇](#infra-robbyoconnor "Infrastructure (Hosting, Build-Tools, etc)") | [<img src="https://avatars0.githubusercontent.com/u/2083930?v=4" width="100px;"/><br /><sub>Bogdan Luca</sub>](https://github.com/lbogdan)<br />[🐛](https://github.com/CompuIves/codesandbox-client/issues?q=author%3Albogdan "Bug reports") [💻](https://github.com/CompuIves/codesandbox-client/commits?author=lbogdan "Code") | [<img src="https://avatars3.githubusercontent.com/u/6177621?v=4" width="100px;"/><br /><sub>Divjot Singh</sub>](http://bogas04.github.io)<br />[💻](https://github.com/CompuIves/codesandbox-client/commits?author=bogas04 "Code") | [<img src="https://avatars3.githubusercontent.com/u/5249539?v=4" width="100px;"/><br /><sub>Jason Nall</sub>](http://www.jsonnull.com)<br />[💻](https://github.com/CompuIves/codesandbox-client/commits?author=jsonnull "Code") |
+| [<img src="https://avatars2.githubusercontent.com/u/2678610?v=4" width="100px;"/><br /><sub>Valentin Hervieu</sub>](http://valentin-hervieu.fr)<br /> |
 <!-- ALL-CONTRIBUTORS-LIST:END -->

src/app/containers/Modal.js

@@ -100,7 +100,7 @@ class ModalContainer extends React.PureComponent {
     overlay: {
       backgroundColor: 'rgba(0, 0, 0, 0.75)',
       overflowY: 'auto',
-      zIndex: 20,
+      zIndex: 30,
     },
     content: {
       position: 'relative',