Implement mixins (as a demo)

[turt2live]

Reference: https://docs.google.com/document/d/1FUVFzTOF4a6XBKVTk55bVRIk4N9u5uZkHS4Rjr_SGxo/edit#

This is a fairly involved thing though. For the purposes of being a demo, we'll have to make sure we still send backwards-compatible events.

• Event type: m.room.message (we're ignoring the desired name change for now)
• [ ] Sending events use mixins (try to be backwards compatible with msgtype where possible)
• [ ] Render all event types using mixins
• [ ] Fall back to current spec (no mixins)