Space-level display name and avatar changes

[gabrc52]

Currently, if you wish to change your display name or avatar in a space, you have to do it in all the rooms that you're in.

Sure, this could be implemented as a client-side feature, but it wouldn't propagate to new rooms you join.

Since the display name and avatar are stored in a m.room.member event, I'm unsure how this would be implemented, but it could be useful to have the ability to "cascade" parts of state events or account data to children of a given space (recursively? probably), in a way that it can be overriden for specific rooms. This might be a way of supporting other space-level settings like #881