Improved markdown mobile editor experience

- Updated styles of codemirror area to be a bit more forefull in taking
up space.
- Added a fullscreen toggle as a backup option.

For #1675

resources/sass/_codemirror.scss

@@ -403,6 +403,7 @@ span.CodeMirror-selectedtext { background: none; }
   left: 0;
   width: 100%;
   height: 100%;
+  margin-bottom: 0;
 }
 
 /**