Dont delete orphan media

This MR aims at not always deleting media when their parent folder gets removed.

Originally we started implementing some hacks in the playlist in case the user would remove a folder for the indexed locations, only to realize it was a mistake and adding it back again.

Now, instead of removing the folder & all the associated media, we will convert the media to external ones, thus keeping their playlist & media group memberships. If the file gets removed from the disk, then the behavior is unchanged and everything will be removed from the database.

This fixes #297 (closed) and will allow development for #292 (closed) to proceed