Bulk Email improvements for release

[brianhw]

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.

[brianhw]

@sarina @flowerhack Please review.

[sarina]

Can you explain how this differs from the original prs?

Also what PRs does this cover?

[brianhw]

@flowerhack It would be particularly helpful if you could try running this branch and just doing a sanity check that the new dash works as you expect.

[flowerhack]

Just finished looking over the dashboard stuff & ran it locally; everything seems to work as expected, so this looks a-OK from where I'm standing

[sarina]

OK makes sense to me