[API-235] Improve library/playlists performance

[raymondjacobson]

Improve library playlists/albums performance

1. Filter on save_type = playlist instead of != track. albums fall under save_type = playlist anyway (checked prod db). This makes use of the index and takes the first query from ~1.5s to 2-300ms.
2. Conditionally join on users
3. Omit tracks in the fullplaylistskeyed query rather than post-facto

Tested:

• Add new test (no coverage before)
• Prod request with replaced URL

[dylanjeffers]

Thank you!

[dylanjeffers]

Nice