Add a command to prune library from artists/albums/tracks with no uploads

Users are usually quite disturbed than removing a file leaves empty artists, albums and tracks behind.

Let's provide a command to help with that process.

changed milestone to %0.19

added AreaBackend ComponentLibrary TypeEnhancement labels

mentioned in commit 5916a1ba

mentioned in merge request !698 (merged)

closed via commit 7bb0fa2e

closed via merge request !698 (merged)

closed via commit 5916a1ba

mentioned in commit d4399723