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/res/layout/feed_item_carousel.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<androidx.recyclerview.widget.RecyclerView
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    android:id="@+id/recycler_view"
+    android:layout_width="match_parent"
+    android:layout_height="wrap_content"
+    android:scrollbars="none"/>