Items with no available cover image will keep being re-requested whenever shown

[dweymouth]

Currently there is no system in place to mark a cover as requested but unavailable. So whenever an entity (artist, album, etc) with no available art is shown on screen, it will re-request the art from the server. We should store the fact that the cover is requested but unavailable so it will not keep retrying.

Ideas:

• Have an in-memory set of unavailable coverArtIDs in the ImageManager - this will obviously not persist after the app is quit
• Storing a zero-size <coverID>.jpg in the local image cache folder. This will mean the image will not be re-requested until the standard expiry for cached covers, even if the app is restarted. Need special handling in ImageManager to detect the zero-size "jpg"s

[dweymouth]

We also may want to treat a 429 (rate limited) error differently from a 404 or other error. May need to update go-subsonic to expose this in the error type returned by the getCoverArt function. See https://github.com/SubmarinerApp/Submariner/issues/174 for more context