Fix for #535:

- hide/show code editor based on markdown editor state (could be CODE or PREVIEW);
- changed styles for code editor;

browser/components/MarkdownEditor.js

@@ -221,7 +221,10 @@ class MarkdownEditor extends React.Component {
         onKeyDown={(e) => this.handleKeyDown(e)}
         onKeyUp={(e) => this.handleKeyUp(e)}
       >
-        <CodeEditor styleName='codeEditor'
+        <CodeEditor styleName={this.state.status === 'CODE'
+            ? 'codeEditor'
+            : 'codeEditor--hide'
+          }
           ref='code'
           mode='GitHub Flavored Markdown'
           value={value}