Both notifyNewTopic and notifyPostChange trigger mention notif

When posting a new topic with a reply, it would trigger two identical
mention notifications. We don't need to trigger it in new topic because
it will be always covered by the post change

(cherry picked from commit 1c0ce37b74195e761ab4d281c41c06f082d1f01b)

Change-Id: I3d266acdf0421fa0b378a08f5c5e46e622f1be3d
Merge "Both notifyNewTopic and notifyPostChange trigger mention notif" into wmf/1.24wmf20