Additional configuration options for auto-join rooms

[clokep]

This adds a couple of additional configuration options for auto-join rooms:

• autocreate_auto_join_rooms_federated to control whether those rooms should be federated
• autocreate_auto_join_room_preset to control the preset when those rooms are created
• auto_join_mxid_localpart to help with rooms that need an invite to be sent to join (e.g. invite only rooms)

This also makes a few tangentially related changes:

• Updates some documentation in the config
• The auto-join rooms now explicitly must be aliases, not IDs (as far as I can tell providing room IDs has been broken for > 5 years)
• Uses some constants in additional places

[erikjohnston]

I understand the config options comments got discussed at length, so I won't weigh in there 🙂 Code LGTM though