Expanded notifications

## Expanded notifications
### Increase user engagement with product and material

### Add option to follow:

- new annotations from group
- new texts annotated by group
- replies from group members to other group members (besides user)
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### From a user:

>I'm checking back in about notifications. I continue to think they would make it much easier to engage new users of Hypothesis in a group project. The existing notification feature, which tells me if someone replies to my annotation, is important so that I don't miss responses to my own work. But I'm looking for a higher level of notification that would alert anyone in a group to a new article being added, or maybe to any new annotation. (These would have to be optional, of course). I think that's the key to engaging people who, like my colleagues in the fake news project, are willing to create a Hyopthesis account but not sufficiently committed to keep a tab open or remember to check the page routinely.

Start by reviewing the existing notification feature and deciding which group events are in scope: new annotations, newly annotated texts, replies between group members, or new articles. Define optional follow settings and acceptance criteria for each event before locating implementation and tests. Done means the agreed notification choices and delivery behavior are documented and validated.

Written by the indexing model from the issue text.