Since we are using EasyMDE now, we do not need to keep the SimpleMDE code anymore.
This PR removes all legacy SimpleMDE code, and makes some related changes:
* `createCommentEasyMDE` can accept native DOM element, and it doesn't need `jQuery.data` to store EasyMDE editor object (as discussed about the frontend guideline).
* introduce `getAttachedEasyMDE` to get the attached EasyMDE editor object, it's easier to find all the usage of EasyMDE.
* rename variable names from `$simplemde` to `easyMDE`, the `$` was incorrect because it is a EasyMDE editor, not a jQuery object.
With this PR, it will be easier to do more refactoring or replacing EasyMDE with other editors.
