Ability to autojoin rooms when you join a community

[ara4n]

Right now we have the ability to autojoin rooms when you register an account on a given HS.

However, it could also be useful (for Slack/Discord style semantics) if you autojoined a set of rooms on joining a given group. We'd need to be careful about being clear why these rooms have been joined though (perhaps we should reconsider grouping RoomList via group?) - and we need to be very careful about abuse (if accepting an invite to a +evil:matrix.org then autojoins you to 200k rooms).

(Split out from #3843)

[turt2live]

#3848 is related to the start chat button - is this correct? Eitherway, this seems like a duplicate of #5244

[ara4n]

so it is.

[ara4n]

fixed broken link