Ability to disable image metadata stripping per guild or channel

This reported bug https://feedback.fluxer.com/p/4435/audio-tag-wasn-t-there-after-downloading-audio-on-fluxer-whe is related, as Fluxer also does a best-effort metadata strip on AV files, based on fluxer_media_proxy/src/mime/registry.rs. The proposed feature could also cover other types of content.