This expands the current shadow-banning feature to be usable via
the admin API and adds documentation for it.
A shadow-banned users receives successful responses to their
client-server API requests, but the events are not propagated into rooms.
Shadow-banning a user should be used as a tool of last resort and may lead
to confusing or broken behaviour for the client.
#8174 removed the `is_guest` parameter from `get_room_data`, at the same time that #8157 was merged using it, colliding together to break unit tests on develop.
This PR removes the `is_guest` parameter from the call in the broken test.
Uses the same changelog as #8174.
