Embeds should also redirect when the topic is merged

[Colonel_mortis]

When topics are merged, the view (manage) requests are redirected, but not any other controller methods, including not embeds. The redirects are really useful, but given that most links are actually in the form of embeds there's a significant gap there.

There's also a lot of room for improvement on stopping people from destructively merging the wrong way on big topics (followers don't get ported across, and are irrecoverably deleted, maybe some other things too). The first post in the topic will always be the oldest, regardless of which was kept, and in almost every case I can think of, that means I would want to keep that post's title and therefore URL as well.