Draft: Display children when viewing Space room

Currently looking at a Space room just displays leave and join events. This makes it display a list of the children of the space.

• It doesn't yet treat subspaces specially.
• It (tries) to find m.space.child events itself instead of using the unstable /hierarchy endpoint (similar to the endpoint for public rooms) to have the homeserver do that.
• AFAICT, Quotient doesn't currently have a way to expose all state events of a given type (m.space.child in this case), so it looks at recent history to get them. I'll try to add support to Quotient for this so spaces that were not recently added show up.
• Space children that we are not a member of display as just the room ID since we don't fetch the room metadata for those yet.
• The via servers are ignored (and thus not used to help find a server that has the rooms).

Contributes to #361 (closed).