Allow changing guild owner in the admin page

Problem

When a user that is the owner over a large or important Guild needs to be banned for abuse or something else, the Guild is threatened.

Proposal

Discord in this case is able to change the owner of the Guild to someone else, and admins on Fluxer self-hosted instances should be able to do the same thing.

Notes

I saw a similar discussion in #1110 which seems to be quite stale now. That discussion is about a much more extensive succession system by the user themselves, which can also be nice but this one is specifically about a forced admin action via the admin system.