Importer updates: watch directories, handle metadata updates

Fix #741 (closed) and #721:

• Updating metadata on existing in-place imported files when performing a scan
• Add a --watch flag on the import_files to react to filesystem events (file creation, update, removal, move)
• Add a --prune flag to easily prune library from dangling metadata
• Updated documentation to reflect the changes (cf http://funkwhale.pages.funkwhale.audio/-/funkwhale/-/jobs/47281/artifacts/docs-review/admin/importing-music.html)

I'd like to see this included in %0.21.1 as it's backward compatible and should considerably improve the UX around importing.

I've tested the --watch flag with beets and Picard, and in both cases, metadata changes applied through these tools were picked immediatly by import_files.