UI for adding/removing channel moderators

[pdpinch]

As a channel moderator, I would like to be able to add or remove moderators who aren't program staff. For example, I might want to add a program graduate to help moderate posts.

3759 addresses the back-end requirements. This issue is to implement the front-end, including a REST API.

acceptance criteria

• [ ] simple micromasters REST API to allow creating and removing moderators for a channel
• [ ] access limited to program staff
• [ ] add/remove moderators by username or email address
• [ ] react front-end for adding and removing moderators

[pdpinch]

This is on hold so we can prioritize other work.

[pdpinch]

I'm not quite ready to close this, but we're probably going to do this in Open Discussions instead of MicroMasters.