Notifications for LMS users

As a teacher, I want to be notified when:

- Students make annotations on assignments so that I can keep up with overall course activity, and answer student questions in a timely manner
- Someone replies to my annotation so that I can facilitate conversations with my students

As a teacher, I would like Hypothesis to send a notification when:
- I want a particular student to know about or reply to an annotation I've made, so I can facilitate the conversation and keep it alive throughout the semester
- I want the whole class to know about an annotation I've made, so I can ensure all of my students are aware of important pieces of the document we're reading

As a student I want to be notified when:

- Someone replies to my annotation, so that I can engage in discussion with my teacher and classmates

As a student, I would like Hypothesis to send a notification when:
- I tag my teacher, so I can ask a question of my teacher as I'm reading
- I tag a classmate, so I can ask them a question, let them know I agree with their point, and generally engage them in conversation about the reading

Some lessons learned from custom notification setups: https://docs.google.com/document/d/1zSkMjYsAT-hA0ZuX2JgsKq-JklgPAKzjuJ6aIIxJdyQ/edit#

Considerations for Canvas:https://github.com/hypothesis/support/issues/112

Previous issue: https://github.com/hypothesis/product-backlog/issues/235

Review the linked custom notification notes, Canvas issue, and previous issue for context. No files, tests, entry points, or concrete completion criteria are mentioned, so the implementation scope remains undefined.

Written by the indexing model from the issue text.