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/header_item.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="utf-8"?>
+<TextView
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:tools="http://schemas.android.com/tools"
+    android:id="@+id/header_title"
+    android:layout_width="match_parent"
+    android:layout_height="wrap_content"
+    android:gravity="start|center_vertical"
+    android:paddingBottom="12dp"
+    android:paddingLeft="16dp"
+    android:paddingRight="16dp"
+    android:paddingTop="12dp"
+    android:textColor="?android:attr/textColorPrimary"
+    android:textSize="16sp"
+    android:textStyle="bold"
+    tools:text="Header"/>