Manage subscriptions

- make use of the SubscriberSegment::status column to keep track of unsubscriptions
- unsubscribed segments now appear grayed out in the Subscribers listing
- added unsubscribed_at after segment names when editing a subscriber
- added date() method for Twig (uses WP's date format / date_offset)
- fixed typo in Form iframe export
- fixed unit test for Newsletters
- updated selection component (JSX) to allow more customization

assets/css/src/admin.styl

@@ -17,3 +17,5 @@
 
 @require 'settings'
 @require 'progress_bar'
+
+@require 'subscribers'

assets/css/src/subscribers.styl

@@ -0,0 +1,3 @@
+#subscribers_container
+  .mailpoet_segments_unsubscribed
+    color: lighten(#555, 33)