Add impersonation mode for usage by staff

[mellowagain]

I propose the implementation of a impersonation mode that will allow a staff member (to be exact, a admin) to impersonate a user both in Bancho and on the frontend.

This will allow a admins to do any actions on behalf of a user, including submission of scores as well as sending chat messages.

The privacy concern is not really valid regarding this suggestion as administrators can already edit everything all over the place, anyways.

There are some situations in the life of a server where impersonation of another user may be the key to getting a rule breaker banned.

This is inspired by Gitlab's impersonation mode:

[48cf]

Actually, this sounds like a great idea to me!@ +1 pls add this to shiro @Marc3842h