Remove old editing code

Change-Id: I3d7f3c3784777a1675878fd22cddd08133f6e78a
Add editor overlay

Basic version, does nothing.

Change-Id: Ib0b144214829b1ebd1867e4c7c77547440cc85fc
Fix header buttons and add "pressed" CSS

Change-Id: I0d2721e91b72017e6268269f06ccdf53ee05918c
Add section loading in editor

Change-Id: Ib21e5c16bff4e549082c996bce59444cc1aa7c27
Tests for Prev/Next, loader, styling fixes

Change-Id: I8a10e47dacb780add6236f56fea35955ea90514e
Caching and saving in EditApi

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: Ia93ec8b501214296fb1bb7c97a65de13194ed829