Playlist upload rate-limits and icon validation

[Toastbrot236]

Playlist creation requests from both LBP1 and 3 are now rate-limited to discourage spamming empty unedited playlists, which has already happened multiple times since Patchwork 1.3 released, so it might become a problem soon. Update requests are also rate-limited but less tightly. Also, both LBP1 playlist upload (create and update) endpoints will now set the request's icon to blank if it is invalid or missing, and I've finally added a few unit tests for playlists, mostly to test icon validation.