Implement users.password.replace_own RPC function

[mathemancer]

We need to implement a users.patch RPC function to replace the POST /api/ui/v0/users/password_change/ endpoint.

The function signature should be:

@rpc_method(name='users.password.replace_own')
@http_basic_auth_login_required
@handle_rpc_exceptions
def replace_own(
        *,
        user_id: int,
        old_password: str,
        new_password: str,
        **kwargs
) -> None:

A user can only use this function to modify their own password.

See the wiki for more context.