Allow Assignee to unassign themselves to issues or only allow assignment to project members
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Problem to solve
Issues can be assigned to users outside of a project.
Further details
When projects are imported (from another GitLab instance, GitHub or other source), GitLab will match on email or related identity (e.g. GitHub). In these cases, it may assign/mention/etc. users who are not part of the project/group.
For public projects, the issues assigned to them will show in their list of issues.
Unfortunately, because the user is not a project/group member, they cannot unassign themselves.
Proposal
Depending on the direction of gitlab-ce#52635, we could do it similar to the idea behind gitlab-ce#41973.
Allow users who are assigned to an issue to edit that specific issue even if they're not a member of the project.
The other option might be to only allow Assignee to be set to members who are in a group/project, but this is probably less preferred because in most cases when doing imports, it's because of migration and not all users will have necessarily been added to the group before import, and cannot be if it's a personal namespace.
Links / references
Originally reported by user (internal): https://gitlab.zendesk.com/agent/tickets/108285