Add config option to make newly registered users auto-join a room

Note for self later: see make_user_admin, its usage in the /register endpoint, and stuff at https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html#auto_join_rooms

Might need to use some parts of make_user_admin to join local rooms, or we could just re-use the join room helper functions.

To limit potential abuse, we'll only allow users to auto-join public rooms.

assigned to @girlbossceo

added enhancement low-priority labels

This is now implemented

closed