Use a custom 'locator' state event rather than `m.room.create`. [rei:wedf/a]

[reivilibre]

This notably lets us re-use existing rooms if desired. I also rip out the parts that store the schedule in the matrix room. These make it unnecessarily confusing as we have 2 sources of truth and provided privacy leaks where some of the information was private (e.g. speaker e-mail addresses).

Some exceptions — room state we keep around:

• new locator, which just serves to correlate rooms with what's in the schedule
• person overrides (after they accept an e-mail invite) — I've rebranded these as overrides to try to make it more obvious why we have them and how they interact with the ones in the underlying schedule.

Stacked PR Chain: rei:wedf/a

PR	Title	Status	Merges Into
#130	Allow disabling IRC bridge by not configuring one, to prevent crash		-
#131	Fix !schedule view falling over with long schedules		#130
#133	(Draft) Try to disconnect the Conference bot from being so reliant on Pentabarf (best effort)		#131
#132	(Draft) Add a JSON schedule loader		#133
#134	(Draft) Fix some misc bugs		#132
#135	(Draft) Disable features relating to Q&A if desired		#134
#136	(Draft) Refactor the backends to be called backends		#135
#137	👉 (Draft) Use a custom 'locator' state event rather than m.room.create.		#136
#138	(Draft) Add a schedule cache as a fallback for if the original schedule goes offline		#137
#139	(Draft) Add a status command to get some information about the bot and its situation		#138

[MatMaul]

Wooot neat that was killing me. One source to rule them all !