Workspace pane was missing a minSize (codesandbox#240)

* The Workspace pane was missing a minSize

...which allowed the view to get squished past a legible width.
Set to same as defaultSize for now.

* go for smaller minSize

Author: brumm

src/app/pages/Sandbox/Editor/index.js

@@ -39,6 +39,7 @@ export default class ContentSplit extends React.PureComponent<Props, State> {
         <SplitPane
           split="vertical"
           defaultSize={16 * 16}
+          minSize={14 * 16}
           style={{ top: 0 }}
           onDragStarted={this.startResizing}
           onDragFinished={this.stopResizing}