Unmerge/Unsplit functionality

[spiros]

I think it would be cool if one could fix an erroneous action of this type by undoing it (and having the merged/split posts acquire again their old address).

[Arantor]

Unsplit is easy enough. Just remerge it.

Unmerge, however, would require storing a duplicate table of references to each topic and message (individually) that is merged. It's a fairly serious undertaking to implement.