Allow for collaboratively edited Page Notes/Annotations within groups

## Feature Request Form
[From Zendesk Ticket 2157 ](https://hypothesis.zendesk.com/agent/tickets/2157)

### Problem you are trying to address with this feature
"it would be great if group members could collaborate on a page note together (similar to how Google Docs allows multiple people to collaborate on one document). Some of my classmates and I are using Hypothes.is to annotate a document together for a project. I created a "main note" to consolidate the gist of everyone's annotations all in one place (which will make turning in our final project easier). However, since I am the one who created the main note, I'm the only one who can edit it. We'd really appreciate all group members being able to edit that main note. Right now, everyone has to either dictate their thoughts to me so I can type them into the main note OR use a separate Google Doc. Having everything in one place would be fantastic."

### Your solution
Page notes created within groups could be authored either (a) by an individual (editable only by that individual) or (b) by the group (editable by anyone who belongs to the group)

No files, tests, or entry points are named. Start by resolving the proposed individual-versus-group authorship behavior and define acceptance criteria for group member editing before locating the implementation.

Written by the indexing model from the issue text.