Bulk Email improvements for release

This PR contains:

* Bulk email authoring, sending, and monitoring on the legacy instructor dash.
* Bulk email authoring and sending on the beta instructor dash.
* Enablement of bulk email authoring and sending for specific courses using admin console. 

Contains the following PRs:
* https://github.com/edx/edx-platform/pull/1215
* https://github.com/edx/edx-platform/pull/1209
* https://github.com/edx/edx-platform/pull/1351
* https://github.com/edx/edx-platform/pull/1322

It does not contain a merge of PR 1260, but the code for that had already been merged into 1209.

The migration of 1322 and 1209 went smoothly.  However, PR 1215 required some changes:

* The branch contained only partial support for disabling buttons for large courses.  This code was broken, and was removed during the merge.
* Changes to scss files needed to support new styling on the instructor dash needed to be manually tweaked.
* Changes to how the HTML Editor is called as an XModule needed to be reverted to bring into line with older XModule code.  

These were addressed in the first commit after the @flowerhack commits.