Fix moderation alerts being sent for boards

When a user is subscribed to both, a topic and the board the topic belongs to and the topic has a moderation action then it would also issue a duplicate (but empty) alert for the board as well

Signed-off-by: Shitiz Garg <mail@dragooon.net>
Merge pull request #2449 from Dragooon/release-2.1

Fix moderation alerts being sent for boards