Adds message to the wiki page creation hook

Hello, this is my first MR to gitlab and is my take on adding the commit message to a wiki page creating hook, as described by #19301 (moved). I'm not sure if I'm testing this in the best place, however I couldn't find a better spot.

The patch actually does more than simply adding the commit message to the web hook, it also sets the message on the object attributes after it is created. I think it makes sense, a smaller alternative would be simply merging the commit message on #hook_attrs. In the end, the simpler alternative seems better, because there is a complex interaction between golumn commit messages and the cached value used as response on #message.