Allow non-admin users to delete review requests

[Zettat123]

Fix #14459

The following users can add/remove review requests of a PR

• the poster of the PR
• the owner or collaborators of the repository
• members with read permission on the pull requests unit

[KN4CK3R]

Could you create tests for the api calls which test the behaviour?

[wolfogre]

BTW, I think it's a design change (even somewhat breaking) instead of bug fixing. I'm not sure if it's a good idea to backport it.

[delvh]

members with read permission on the pull requests unit

Erm… read?
Why not write?

[Zettat123]

members with read permission on the pull requests unit

Erm… read? Why not write?

In my view, a user who can read the PR is also likely to know who should/shouldn't review the PR. Also, I followed @lunny 's idea in #14459 (comment) where the read permission was mentioned. So I only check the read permission here.

[lunny]

members with read permission on the pull requests unit

Erm… read?

Why not write?

Read permission of issue or pr units can create issue or pr.

[silverwind]

Ready for merge?