ReplayGain support + audio normalization (web player) (#1988)

* ReplayGain support

- extract ReplayGain tags from files, expose via native api
- use metadata to normalize audio in web player

* make pre-push happy

* remove unnecessary prints

* remove another unnecessary print

* add tooltips, see metadata

* address comments, use settings instead

* remove console.log

* use better language for gain modes

ui/src/App.js

@@ -26,6 +26,7 @@ import {
   albumViewReducer,
   activityReducer,
   settingsReducer,
+  replayGainReducer,
   downloadMenuDialogReducer,
 } from './reducers'
 import createAdminStore from './store/createAdminStore'
@@ -59,6 +60,7 @@ const adminStore = createAdminStore({
     listenBrainzTokenDialog: listenBrainzTokenDialogReducer,
     activity: activityReducer,
     settings: settingsReducer,
+    replayGain: replayGainReducer,
   },
 })