Implement a client-side user filter for the Notebook

[lyzadanger]

Add the ability to filter a group's (course's) annotation by user/author. Note that this first implementation will be client-side, meaning we'll build the list of available users from the annotation set we have available.

This being the first filter we're adding, there's likely some infrastructure to establish—state, thread-building, etc.—that we'll want to sort out at this point. With luck, additional filters will be faster to add.

[lyzadanger]

Based on what the focus of this task is, I think I'm going to call it "done." A user filter, client-side, has been implemented, and the mechanics needed to "set filters" are in place.