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/App.js

@@ -20,6 +20,7 @@ import {
   themeReducer,
   addToPlaylistDialogReducer,
   expandInfoDialogReducer,
+  listenBrainzTokenDialogReducer,
   playerReducer,
   albumViewReducer,
   activityReducer,
@@ -54,6 +55,7 @@ const App = () => (
         theme: themeReducer,
         addToPlaylistDialog: addToPlaylistDialogReducer,
         expandInfoDialog: expandInfoDialogReducer,
+        listenBrainzTokenDialog: listenBrainzTokenDialogReducer,
         activity: activityReducer,
         settings: settingsReducer,
       },