Send an email to the user when a recording is done processing

Had to add an email attribute to users to store their email to send the
notification. Will only send it for user rooms since they are the only
ones we know who to notify.

test/mailers/previews/notification_mailer_preview.rb

@@ -0,0 +1,6 @@
+# Preview all emails at http://localhost:3000/rails/mailers/example_mailer
+class NotificationMailerPreview < ActionMailer::Preview
+  def recording_ready_email_preview
+    NotificationMailer.recording_ready_email(User.first)
+  end
+end