Feature: Add online status to each user (#452)

* Feature: Add online status to each user
* Add OnlineStatusable, add availability status to thumbnail

app/models/concerns/availability_statusable.rb

@@ -0,0 +1,11 @@
+module AvailabilityStatusable
+  extend ActiveSupport::Concern
+
+  def online?
+    ::OnlineStatusTracker.subscription_count(pubsub_token) != 0
+  end
+
+  def availability_status
+    online? ? 'online' : 'offline'
+  end
+end