Implement users.patch RPC function

[mathemancer]

We need to implement a users.patch RPC function to replace the PATCH /api/ui/v0/users/{userId}/ endpoint.

The function signature should be:

@rpc_method(name='users.patch')
@http_basic_auth_login_required
@handle_rpc_exceptions
def patch(
        *,
        user_id: int,
        user_info: SettableUserInfo,
        **kwargs
) -> UserInfo:

A superuser can patch anyone's user, and a user can patch their own user.

See the wiki for more context.