Notifications for LMS users

We have [an existing issue for "expanded notifications"](https://github.com/hypothesis/product-backlog/issues/235) for our non-LMS offering, but the LMS app does not have any notifications built in.

Some questions to consider:

**What should users notified about?**
Might include (among other things):
- new top-level annotations from other group members
- new replies (to the user, or to other group members)

**Where and how should users be notified?**
In app? Via email? Both? (We don't currently collect email addresses for LMS users)

**How do we avoid creating too much "noise" for users?**
If a professor is notified every time an annotation is made, and a class of 20 students makes 5 annotations each, that's 100 notifications for the professor. This can get unwieldy quickly.

...I am sure there are other things we need to consider, but this issue is serving as a placeholder.