Add an endpoint to recheck an entire folder for large images

[manthey]

This would check if any files could be used as large images that aren't marked as such, and would check if existing large_images are still readable, and, if not, unmark and try to remark them. It should have a recurse option.

Ideally, this would be exposed as a local job so that it can be cancelled.

[manthey]

This is related to #873.