Moderation for LMS app

Originally, our moderation capabilities in the LMS app were quite limited. A teacher could hide flagged annotations, but:

- Since we don't collect email addresses, teachers did not receive notifications for flagged annotations.
- If there was more than one teacher per course, only one of those teachers would have moderation capabilities.
- There was no way to change which teacher was the moderator - the privilege would just be assigned to the first teacher user to launch a Hypothesis assignment in the course.

As we build out our groups capability for the LMS app, this bit of functionality will be going away. ([See discussion in Slack](https://hypothes-is.slack.com/archives/CBN3DGW02/p1580142327025200))

The gist is: our current implementation of "first-teacher-is-moderator" will be difficult and hack-y to maintain when there are multiple groups per course. Rather than expending a ton of effort to keep a less-than-ideal implementation in place, we're going to remove the flagging/moderation function and bring it back to the LMS at a later time.