REFACTOR: Move the `discourse-markdown` engine out of the addon repo

The reasoning here is that we want to make a unique bundle for this
folder and the default approach is to include everything in `addon`.

app/assets/javascripts/markdown-it-bundle.js

@@ -1,20 +1,20 @@
 //= require markdown-it.js
-//= require ./pretty-text/addon/engines/discourse-markdown/helpers
-//= require ./pretty-text/addon/engines/discourse-markdown/mentions
-//= require ./pretty-text/addon/engines/discourse-markdown/quotes
-//= require ./pretty-text/addon/engines/discourse-markdown/emoji
-//= require ./pretty-text/addon/engines/discourse-markdown/onebox
-//= require ./pretty-text/addon/engines/discourse-markdown/bbcode-block
-//= require ./pretty-text/addon/engines/discourse-markdown/bbcode-inline
-//= require ./pretty-text/addon/engines/discourse-markdown/code
-//= require ./pretty-text/addon/engines/discourse-markdown/category-hashtag
-//= require ./pretty-text/addon/engines/discourse-markdown/censored
-//= require ./pretty-text/addon/engines/discourse-markdown/table
-//= require ./pretty-text/addon/engines/discourse-markdown/paragraph
-//= require ./pretty-text/addon/engines/discourse-markdown/newline
-//= require ./pretty-text/addon/engines/discourse-markdown/html-img
-//= require ./pretty-text/addon/engines/discourse-markdown/text-post-process
-//= require ./pretty-text/addon/engines/discourse-markdown/upload-protocol
-//= require ./pretty-text/addon/engines/discourse-markdown/inject-line-number
-//= require ./pretty-text/addon/engines/discourse-markdown/resize-controls
-//= require ./pretty-text/addon/engines/discourse-markdown/d-wrap
+//= require ./pretty-text/engines/discourse-markdown/helpers
+//= require ./pretty-text/engines/discourse-markdown/mentions
+//= require ./pretty-text/engines/discourse-markdown/quotes
+//= require ./pretty-text/engines/discourse-markdown/emoji
+//= require ./pretty-text/engines/discourse-markdown/onebox
+//= require ./pretty-text/engines/discourse-markdown/bbcode-block
+//= require ./pretty-text/engines/discourse-markdown/bbcode-inline
+//= require ./pretty-text/engines/discourse-markdown/code
+//= require ./pretty-text/engines/discourse-markdown/category-hashtag
+//= require ./pretty-text/engines/discourse-markdown/censored
+//= require ./pretty-text/engines/discourse-markdown/table
+//= require ./pretty-text/engines/discourse-markdown/paragraph
+//= require ./pretty-text/engines/discourse-markdown/newline
+//= require ./pretty-text/engines/discourse-markdown/html-img
+//= require ./pretty-text/engines/discourse-markdown/text-post-process
+//= require ./pretty-text/engines/discourse-markdown/upload-protocol
+//= require ./pretty-text/engines/discourse-markdown/inject-line-number
+//= require ./pretty-text/engines/discourse-markdown/resize-controls
+//= require ./pretty-text/engines/discourse-markdown/d-wrap