Disallow assigning others as reviewer/committer on open patches

[JelteF]

Many people use assign themselves to open entries as a reviewer/committer as a sort of bookmarking system for open patches they are interested in. Currently it's allowed for others to assign anyone as a reviewer/committer, thus effectively modifying people their bookmarking system. This is especially made worse because you can sign up in your profile to receive commitfest app updates for patches that you are reviewer/committer of.

Assigning reviewers/committers to closed patches should still be allowed though. When committing, reviewers are sometimes added by the committer to reflect the same as their Reviewed-By footer, even if those reviewers didn't explicitly sign up as a reviewer on the commitfest app for this patch.