Allow hook subscribers to also overwrite link to title

Flow, for example, has those totally illegible Titles like
Topic:S0gx52fzru1680ib - instead, we'd rather use the topic
title content as link text.

Change-Id: I30b3bbb2c5ef131f069a5aabfad8b88bc2cf09df
Merge "Allow hook subscribers to also overwrite link to title"