Implement new feed and subscriptions groups

- Introduce Groupie for easier lists implementations
- Use some of the new components of the Android Architecture libraries
- Add a bunch of icons for groups, using vectors, which still is
compatible with older APIs through the compatibility layer

app/src/main/AndroidManifest.xml

@@ -74,6 +74,7 @@
 
         <service android:name=".local.subscription.services.SubscriptionsImportService"/>
         <service android:name=".local.subscription.services.SubscriptionsExportService"/>
+        <service android:name=".local.feed.service.FeedLoadService"/>
 
         <activity
             android:name=".PanicResponderActivity"