core: space hierarchy

[avdb13]

Currently the Matrix protocol allows nesting of spaces without boundary, possibly allowing for recursive loops and performance issues on the client-side down the road.

This issue tackles a set hierarchy of spaces defined as part of Commune itself:

• level 1: space
• level 2: topic
• level 3: channel

All of these are just Matrix rooms behind the scenes, with the first two level being limited to state events.

I also propose differentiating between standalone rooms and spaces altogether with m.direct instead of the Matrix approach which relies on it for PMs between two entities only.