Use board Title instead of topic Title

Since we've recently introduced a topic namespace, the notification looked like:
"Username thanked you for your comment in "Topic title" on Topic:UUID"

We want the board name instead of the link to the topic page, like:
"Username thanked you for your comment in "Topic title" on Talk:Boardname"

Change-Id: If45c01be3b0a9d67572932253a0103a2480fc823