Merge topics as well as other things in ContentMerger

It's not really clear how to contain it neatly (visitors don't count as neat) – this is sort of a compromise; the types are basically used as symbols, with all the possibilities kept local to this file (so it's easy to reason about whether any have been missed), but that's all it really achieves.

Of course, this specific line could go back to topicsMergeStrategy==MergeStrategy.REPLACE, which is possibly more efficient? But that seems more surprising somehow :\