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/drawable/dashed_border_dark.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape xmlns:android="http://schemas.android.com/apk/res/android">
+    <stroke
+        android:width="1dp"
+        android:color="@color/dark_border_color"
+        android:dashGap="4dp"
+        android:dashWidth="4dp"/>
+</shape>