Prevents the silencing of notifications that are actively deployed. (#454)

* Renaming 'active' to 'notify' as this is clearer and more aligned to what this value is actually controlling. 'active' is now a property that depends on whether any endpoints were found to be using the certificate. Also added logic for issue #405 disallowing for a certificates' notifications to be silenced when it is actively deployed on an endpoint.

* Adding migration script to alter 'active' column.

lemur/notifications/service.py

@@ -160,8 +160,7 @@ def _is_eligible_for_notifications(cert):
     :param cert:
     :return:
     """
-    # inactive certificates are not notified.
-    if not cert.active:
+    if not cert.notify:
         return
 
     now = arrow.utcnow()