Allow transfer of model ownership

Cases where this would be useful:

• Someone doesn’t want to maintain or update a model anymore, but another will

• A model was uploaded as a mirror before the main author joined the site, and now it’d be best to transfer to them

• Someone has made another account to thoroughly separate content types, like NSFW and SFW, especially given the new Green site.

This could be merged with others’ ideas for allowing multiple owners, by allowing for shifting of primary ownership, or the uploader leaving the group.

(Note I just thought of, for if it’s helpful: if this the ownership group idea is difficult on a technical level because of unique account identifiers, a work around might be to make a form of pseudo-account with ownership of a page, but with a hashed password linked to the group members’ accounts so that when they make a change to a page it goes through the group pseudo-account as a proxy. I have no idea what the actual backend looks like, but the thought just popped into my head as I vaguely recall seeing that issue somewhere else.)