Allow users to self-request a PR review (#19030)

Allow users to self-request a PR review (#19030)

The review request feature was added in https://github.com/go-gitea/gitea/pull/10756,
where the doer got explicitly excluded from available reviewers. I don't see a
functionality or security related reason to forbid this case.

As shown by GitHubs implementation, it may be useful to self-request a review,
to be reminded oneselves about reviewing, while communicating to team mates that a
review is missing.

Co-authored-by: delvh <dev.lh@web.de>