Limit media:remove_remote to toots that have not been boosted locally

[fwenzel]

Semi-related to #3904:

Clearing out cached remote media is neat to save disk space (or S3 bill), but will, of course, hide remote media in posts that have been boosted locally.

Is there a way to exclude media from posts that have been boosted locally, thus reducing the likelihood of encountering "missing" media when scrolling through a user's timeline?

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• I searched or browsed the repo’s other issues to ensure this is not a duplicate.
• This bug happens on a tagged release and not on master (If you're a user, don't worry about this).