VC/Zoom: Speed up registrant sync for large events

[moliholy]

Enabling auto-registration on a Zoom room, and syncing a single registration, resolved every registrant of the event against the Zoom API inside the request. On events with hundreds of registrants outside the Zoom account this meant hundreds of API calls per request, and the request timed out.

This PR:

• Stops resolving every other registrant against Zoom when syncing a single registration; duplicates are now detected from the registration email and from the users who own that email or the registrant's Zoom account.
• Moves the initial backfill (room creation, enabling auto-registration, selecting more registration forms) to a Celery task launched after commit when it covers 5 or more registrants, and tells managers it runs in the background. Smaller backfills, and every backfill in DEBUG mode, still run within the request.
• Stops a running background backfill before its next batch once auto-registration is disabled on the room or the room is deleted.

[ThiefMaster]

How badly will things break if you enable autoreg (ie the task is triggered) and then disable it again, before that task finished? :)

[moliholy]

They'd overlap. To mitigate that, I was thinking of adding a check that verifies that auto_registration is on before onboarding more users (in every 30-user batch I mean). What do you think?

[moliholy]

I took the liberty of doing so in f030dc1. Anyway it doesn't hurt.