[Task] Set mosaic creation as backend process

[mradamcox]

Describe the task
Currently, mosaic for a volume is generated with a one-off terminal command. The same logic should be hooked up to a celery task, and somehow triggered from the front-end, either automatically or with a button.

Completion criteria
One way this could be handled is every time the multimask is saved, then the mosaic json is regenerated. This isn't ideal, as it would be advisable for a user to save the multimask more than once during an editing session, and triggering the mosaic json creation again a second time before the first request has finished is not a clean solution.

For the completion of this ticket, I think an admin-only button in the multi-trim interface is good enough for now.

[mradamcox]

This is now funded, and will fit within a larger update to the mosaic processing backend. For this particular ticket, I think the most important task will be the creation of a second celery systemd service that is only used to run mosaic processing, and has a very low concurrency setting, probably 2, maybe just 1, so that mosaics can be queued in celery without all running at once.