Send notifications to partecipants in issue comments (#1217)

* Send notifications to partecipants in issue comments

Closes #1216

Includes test (still failing)

* Do not include "labelers" to participants

Fix test to expect what GetParticipants return

models/fixtures/issue.yml

@@ -8,7 +8,7 @@
   content: content1
   is_closed: false
   is_pull: false
-  num_comments: 0
+  num_comments: 2
   created_unix: 946684800
   updated_unix: 978307200