Improve the website UI

- Replace the add button with a clickable div
- Make the id a link instead of div content
Improve ContentController

- Re-enable the save button after unsuccessful save
- Update location when saving
- Cleanup the code
Make index.jsp validate as HTML5
Fix JSON serialization

- Add serializeAsArray and arrayKeys to the JSONProvider
  configuration, to make serialization of a single element
  array work correctly.
- Remove unnecessary logic from index.js