Migration: Re-use async batch infrastructure

[obenland]

One thing I noticed while testing these changes is that we'll end up with a lot of activitypub_async_batch events that kind of obscure what they do until you look at the args for the individual jobs. Not sure how we feel about that.
Maybe we should just keep it to batches and not move the async_migration job?

Proposed changes:

• Updates migration jobs to re-using the Scheduler::async_batch infrastructure
• Update tests to account for the above changes.

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Go to '..'

[pfefferle]

@obenland what about this PR?

[obenland]

Depends on #1521.

[obenland]

@pfefferle This should be ready for review