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/fragment_subscription.xml

@@ -1,7 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <RelativeLayout
     xmlns:android="http://schemas.android.com/apk/res/android"
-    xmlns:app="http://schemas.android.com/apk/res-auto"
     xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="match_parent">
@@ -9,11 +8,10 @@
     <androidx.recyclerview.widget.RecyclerView
         android:id="@+id/items_list"
         android:layout_width="match_parent"
-        android:layout_height="wrap_content"
+        android:layout_height="match_parent"
         android:scrollbars="vertical"
         android:visibility="gone"
-        tools:listitem="@layout/list_channel_item"
-        tools:visibility="visible"/>
+        tools:listitem="@layout/list_channel_item"/>
 
     <!--ERROR PANEL-->
     <include