Support public room creation without address; make room address optional.

Seemingly it’s not possible for users to create public rooms when `alias_creation_rules` are set. This also affects public rooms being created inside of public spaces. _This is an enormous problem for us as an educational institution._

This problem can easily be solved by making the address field in the room creation dialog optional.

Telling each and every user how to change the `join_rule` of private rooms to public and adapt the `room_history` setting is unfortunately not feasible. There needs to be a better solution; making the room address optional could be exactly that.

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Element-Web/Element-Desktop `v1.7.31` and below.