FEATURE: New markdown editor re-written in Ember

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.

app/assets/javascripts/main_include.js

@@ -75,6 +75,7 @@
 //= require ./discourse/views/header
 //= require ./discourse/dialects/dialect
 //= require ./discourse/lib/emoji/emoji
+//= require ./discourse/lib/emoji/emoji-groups
 //= require ./discourse/lib/emoji/emoji-toolbar
 //= require ./discourse/views/composer
 //= require ./discourse/lib/show-modal