* Don't save artwork for permanent items (file + Spotify) to the stash. The
stash is only for short term artwork.
* If a request comes with a different max_w/max then search the online source
again.
* Make artwork requests thread-safe by mutex protecting the search history.
* Add config option
This adds a new settings component for user configurable options that
can be changed through the JSON API.
The settings are stored in the admin db table and not in the conf-file.
