Annotate 1:1 with another user

### Problem you are trying to address with this feature

Two of the most common use cases for groups among educators are:
- Creating a group for a class to collaboratively annotate in
- Creating a 1:1 channel to annotate with individual students (usually for the purposes of grading & assignment feedback)

The second use case requires a laborious workflow for educators - they are required to create as many groups as there are students in their class, and individually invite each student to join.

_From Dan's issue, which was closed in favor of this one:_

Users in many contexts are asking for the ability to annotate only to another single user, as opposed to within a group or the general public.

- Teachers want to respond privately via annotation on a student's work.
- Researchers want to be able to annotate a private note to the author of a paper (same user story as above).
- Teachers want to respond privately to a students annotation.
- Users want to respond to an annotation but only to the annotations author (same user story as above).
- Users want to privately annotate to another user who is neither the author of a document, nor the author of an existing annotation. (essentially like a private @mention).
- These modes would be valuable in a wide range of situations from the open web, to dedicated teaching and peer review contexts.

They would fundamentally expand on the three major modes of annotation we support now (personal, private group, and public) adding a powerful fourth category (1:1).

### Your solution
Establish a "Post" paradigm that would let users specify another user, or the author of a document specifically (if that author's username is made available to the client).

### User requests

- [https://hypothesis.zendesk.com/agent/tickets/2167](https://hypothesis.zendesk.com/agent/tickets/2167)
- [https://hypothesis.zendesk.com/agent/tickets/2225](https://hypothesis.zendesk.com/agent/tickets/2225)
- https://hypothesis.zendesk.com/agent/tickets/3052
- https://hypothesis.zendesk.com/agent/tickets/3134

Start by reviewing the existing personal, private-group, and public annotation modes described in the issue, then map each requested 1:1 use case to the proposed "Post" paradigm. Done means defining a workable feature that supports annotating to another user or a document author without requiring one group per student.

Written by the indexing model from the issue text.