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.

test/javascripts/acceptance/category-edit-test.js.es6

@@ -34,7 +34,7 @@ test("Change the topic template", assert => {
 
   click('.edit-category');
   click('.edit-category-topic-template');
-  fillIn('.wmd-input', 'this is the new topic template');
+  fillIn('.d-editor-input', 'this is the new topic template');
   click('#save-category');
   andThen(() => {
     assert.ok(!visible('#discourse-modal'), 'it closes the modal');