Start transforming m.room.power_levels to their new format.

Update content[userId] to content.users[userId]. Update content.default
to content.users_default. Remove required_power_level.
Update unit tests to use the new m.room.power_levels event format.
Remove m.room.ops_levels
Remove references to send_event_level and add_state_level
Fix bug when redacting events.
Merge pull request #1 from matrix-org/synapse-0.5.0

Synapse 0.5.0 Compat Fixes