Improve Sidekiq queue efficiency  #1381

Feature request ✨

Currently classroom uses sidekiq queues inefficiently.
This is our current queue setup:
https://github.com/education/classroom/blob/4f9db7e412c57ce8aeb328a355b223e0a7254546/config/sidekiq.yml#L7-L14

This is what the Sidekiq docs say about having lots of queues:

I don't recommend having more than a handful of queues. Lots of queues makes for a more complex system and Sidekiq Pro cannot reliably handle multiple queues without polling.
https://github.com/mperham/sidekiq/wiki/Advanced-Options#queues

One way we could improve this would be to use the regular 3 queues (critical, default, and low).