Listenbrainz scrobbling (#1424)

* Refactor session_keys to its own package

* Adjust play_tracker

- Don't send external NowPlaying/Scrobble for tracks with unknown artist
- Continue to the next agent on error

* Implement ListenBrainz Agent and Auth Router

* Implement frontend for ListenBrainz linking

* Update listenBrainzRequest

- Don't marshal Player to json
- Rename Track to Title

* Return ErrRetryLater on ListenBrainz server errors

* Add tests for listenBrainzAgent

* Add tests for ListenBrainz Client

* Adjust ListenBrainzTokenDialog to handle errors better

* Refactor listenbrainz.formatListen and listenBrainzRequest structs

* Refactor agent auth_routers

* Refactor session_keys to agents package

* Add test for listenBrainzResponse

* Add tests for ListenBrainz auth_router

* Update ListenBrainzTokenDialog and auth_router

* Adjust player scrobble toggle

ui/src/personal/Personal.js

@@ -6,6 +6,7 @@ import { SelectTheme } from './SelectTheme'
 import { SelectDefaultView } from './SelectDefaultView'
 import { NotificationsToggle } from './NotificationsToggle'
 import { LastfmScrobbleToggle } from './LastfmScrobbleToggle'
+import { ListenBrainzScrobbleToggle } from './ListenBrainzScrobbleToggle'
 import config from '../config'
 
 const useStyles = makeStyles({
@@ -25,6 +26,7 @@ const Personal = () => {
         <SelectDefaultView />
         <NotificationsToggle />
         {config.lastFMEnabled && <LastfmScrobbleToggle />}
+        {config.devListenBrainzEnabled && <ListenBrainzScrobbleToggle />}
       </SimpleForm>
     </Card>
   )