(Story 486) Add new editor

* Remove old editing code
* Disable desktop editor (action=edit) altogether
* Add editor api and overlay

When a section is modified in the editor, it is staged to be saved after
pressing the Save button. Each staged section is, for the time being, saved
using a separate AJAX request.

Change-Id: Ifaba7cb8ca6d9ae225c346e75e72110cd8a66f94