Improvements all over the app

1. InlineType model no longer needed. A setting called
   INLINE_MEDIA_TYPES indicates what inline types have to be
   available in dropdown lists.
2. Customizable size classes on a per app.model basis through
   INLINE_MEDIA_CUSTOM_SIZES, now change the size classes
   available in the dropdown select element.
3. Button 'add' inserts the <inline> element in the cursor
   position in the textarea.
4. More unittests and less spanglish in the docs.