Collaboration by forking/merging annotations

[philippotto]

In the context of proofreading, we should re-evaluate this idea. I wrote a design doc draft here which is more up to date.

Old issue content:

For a public annotation, users can do "copy to my account" to make changes (e.g., fixes). However, there is no way to stream these changes back in a pull-request fashion.

Currently, one has to send a mail with the "forked" annotation and the owner has to re-apply these changes manually.

Different possibilities to better support this use case could be:
- allow to create some kind of "pull request" which the owner can accept
- support "merging" multiple PRs which are based on a common base version (but there are no conflicts)
- support resolving conflicts if the PRs don't agree in certain aspects

Tangentially related: It would be cool to see a list of "forks" of one annotation. Only if the forker has allowed this probably.

[philippotto]

See design doc: https://www.notion.so/scalableminds/Design-Doc-Collaborative-Forking-Proofreading-52f58094ea1544739371adab4a800cce

[philippotto]

Closing in favor of #4259.