Fix adding excluded users to the private room sharing tables when joining a room (#11143)

* We only need to fetch users in private rooms

* Filter out `user_id` at the top

* Discard excluded users in the top loop

We weren't doing this in the "First, if they're our user" branch so this
is a bugfix.

* The caller must check that `user_id` is included

This is in the docstring. There are two call sites:
- one in `_handle_room_publicity_change`, which explicitly checks before calling;
- and another in `_handle_room_membership_event`, which returns early if
  the user is excluded.

So this change is safe.

* Test joining a private room with an excluded user

* Tweak an existing test

* Changelog

* test docstring

* lint