Story 1521: Allow switching from VisualEditor to Wikitext editor

Also had to clean up a lot of header CSS in the process.

Also add tests to show expected behaviour of the toggle button in
the VisualEditor overlay.

Change-Id: I6e289d02612eb2093dd7ef0a4026b2dfce493ac0
Merge "Story 1521: Allow switching from VisualEditor to Wikitext editor"