Make all space previews work

[squahtx]

To make space previews work, m.room.guest_access {guest_access: "can_join"} and m.room.history_visibility {history_visibility: "world_readable"} events need to be sent in all (sub)spaces.

This was partially done in #88, but that PR neglected to set m.room.guest_access for auditorium subspaces.

TODO: check if m.room.history_visibility is actually required to make space previews work.

[reivilibre]

@squahtx Do you know why m.room.guest_access is needed? I'm looking through Synapse (and the spec... which is underspecified :/) and can't see where it comes into play.

[squahtx]

I can't remember, I'm afraid. It may be to satisfy logic on Element Web?

[turt2live]

Guest access allows anonymous viewers to view the room, as they otherwise would get told to go away (slightly awkward because an authenticated user could see the room, but guest users would be declined 🤷 )