Allow admins to reset a user name or avatar

[cellio]

Sometimes a user will choose a name, avatar, or profile "about" text that is rude in some way, possibly even violating our code of conduct. It would be helpful if we could force-reset them in those cases.

Because profiles are network-wide (you can't have different names on different communities), I'm currently proposing this as an admin tool to avoid cross-community issues. But I'm open to making it a mod tool if that makes more sense or admin-only would be harder.

Either way, we should preserve the previous values in history somewhere (accessible through the UI for admins) and log the event. It should always be possible to audit such events if there's a complaint.

[cellio]

I set priority to low because it's an infrequent event and we can ask devs to do it through the UI, but it would be better to have a few more people have the ability to respond if there's an issue.

[cellio]

Basically a duplicate of #766.