New assets for Chrome/Firefox extension (codesandbox#4084)

* Update description

* Delete still2.png

* Updated screenshot

* Delete icon48.png

* Delete icon16.png

* Delete icon128.png

* Updated Logo

* Update Readme.md

Author: garethx

packages/chrome-extension/Readme.md

@@ -1,6 +1,6 @@
 # Browser Extension
 
-![Image](https://user-images.githubusercontent.com/1051509/46907462-fc0ef600-cf12-11e8-8d73-05b95577364e.png)
+![Image](https://github.com/codesandbox/codesandbox-client/blob/garethx-addon/packages/chrome-extension/assets/still2.png)
 
 [![available in the chrome store](https://developer.chrome.com/webstore/images/ChromeWebStore_Badge_v2_206x58.png)](https://chrome.google.com/webstore/detail/jkhbnhagngalpojoeijaleemepfpefmp)
 [Download for firefox](https://addons.mozilla.org/en-US/firefox/addon/open-in-codesandbox/)

packages/chrome-extension/assets/icon128.png

@@ -3,7 +3,7 @@
   "name": "Open in CodeSandbox",
   "short_name": "CodeSandbox",
   "version": "1.1.0",
-  "description": "Adds an open in CodeSandbox button to Github Repos",
+  "description": "Adds an 'Open in CodeSandbox' button to GitHub repos",
   "homepage_url": "https://codesandbox.io",
   "icons": {
     "16": "./assets/icon16.png",