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/picker_icon_item.xml

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="utf-8"?>
+<ImageView
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:tools="http://schemas.android.com/tools"
+    android:id="@+id/icon_view"
+    android:layout_width="match_parent"
+    android:layout_height="wrap_content"
+    android:background="?attr/selectableItemBackground"
+    android:focusable="true"
+    android:minHeight="24dp"
+    android:minWidth="24dp"
+    android:padding="8dp"
+    android:scaleType="centerInside"
+    tools:ignore="ContentDescription"
+    tools:src="?attr/rss"/>