Implement UI for subscriptions import/export

- Nice and easy to use import/export options in the subscriptions fragment
- Includes instructions for each service (in the import fragment/screen)

app/src/main/res/layout/fragment_subscription.xml

@@ -9,10 +9,12 @@
     <android.support.v7.widget.RecyclerView
         android:id="@+id/items_list"
         android:layout_width="match_parent"
-        android:layout_height="match_parent"
+        android:layout_height="wrap_content"
         android:scrollbars="vertical"
         app:layoutManager="LinearLayoutManager"
-        tools:listitem="@layout/list_channel_item"/>
+        android:visibility="gone"
+        tools:listitem="@layout/list_channel_item"
+        tools:visibility="visible"/>
 
     <!--ERROR PANEL-->
     <include
@@ -21,6 +23,7 @@
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_centerHorizontal="true"
+        android:layout_below="@id/items_list"
         android:layout_marginTop="50dp"
         android:visibility="gone"
         tools:visibility="visible"/>
@@ -31,6 +34,7 @@
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_centerInParent="true"
+        android:layout_below="@id/items_list"
         android:layout_marginTop="50dp"
         android:visibility="gone"
         tools:visibility="visible"/>