Add notifier back to CI.

CHANGELOG.md

+- Role: notifier
+  - Added notifier back to continuous integration.
+
 - Role: discovery
   - Added `DISCOVERY_REPOS` to allow configuring discovery repository details.
 

playbooks/edx-east/edx_continuous_integration.yml

@@ -30,9 +30,7 @@
     - oraclejdk
     - elasticsearch
     - forum
-# Removing until Notifier is fully fixed.
-# Can be uncommented once EDUCATOR-1594 has been resolved.
-#    - { role: notifier, NOTIFIER_DIGEST_TASK_INTERVAL: "5" }
+    - { role: notifier, NOTIFIER_DIGEST_TASK_INTERVAL: "5" }
     - { role: "xqueue", update_users: True }
     - role: xserver
       when: XSERVER_GIT_IDENTITY|length > 0