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/stylesheets/mobile/user.scss

@@ -67,10 +67,6 @@
     display: none;
   }
 
-  .pagedown-editor {
-    width: 100%;
-  }
-
   textarea {width: 100%;}
 }