Note this commit leaves out the biggest occurrence of the editor
which is the post/topic composer.
To avoid major breakage, this replaces it everywhere else it was
used:
* User preferences (About Me)
* Admin Customizations > Text Content
* Category Templates
* Editing Queued Posts
A future commit will replace the main composer with this editor
and will remove the unused pagedown code.
Robin Ward
Neil Lalonde
Régis Hanol