Remove concept of reqired power levels, something similiar can be done
using the new power level event.

Don't cache isWebRTCSupported because whether webRTC is supported might change part-way through the page's lifecycle if your webrtc support comes from some kind of injected content script (hello OpenWebRTC Sarafi extension)

`prev_state` is now a list of previous state ids, similiar to
prev_events. `replace_state` now points to what we think was replaced.

Swap from using raw m.room.member events for room members to using actual RoomMember objects, so User objects can be tacked on. Update tests.