Verify mount in BackgroundMoveWorker before using `public_send`

What does this MR do and why?

This is mostly safe anyway, as it doesn't pass any arguments, but we should still check that the given file_field (mount point) is actually an uploader of the correct type before we call it.

A Sidekiq job class that takes a method name as an argument and calls that method (with public_send on an object) is a security risk. It's not typically directly exploitable, but it can be used to escalate from Redis injection to arbitrary code execution (see #371098 (closed) for an example).

We should remove public_send from Sidekiq workers where possible, and where not possible, validate the method name we're passing carefully. I found these workers that disable the Security/PublicSend cop:

Split from !103053 (closed). For #371470 (closed).

How to set up and validate locally

Mostly just specs.

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