A content editor should not be able to re-use artifact files

[MegaKeegMan]

The following artifact node types are currently using media reference fields, but maybe should not:

• [x] video
• [x] audio
• [x] image
• [x] pdf document

We should either find a way to hide these from the media library or just switch them away from media entirely (leaning into the latter). The goal is to simplify the permissions around private content and files.

[MegaKeegMan]

Noting that the video artifact field was never actually transitioned to a media entity reference, but may still need some slight cleanup