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/admin/templates/site-text-edit.hbs

@@ -2,7 +2,7 @@
 <p class='description'>{{model.description}}</p>
 
 {{#if model.markdown}}
-  {{pagedown-editor value=model.value}}
+  {{d-editor value=model.value}}
 {{/if}}
 {{#if model.plainText}}
   {{textarea value=model.value class="plain"}}