I wanted to fix #885, which I did.
However it appeared that Page.get_absolute_url was a complete mess.
So I ended up fixing that, by moving page-path (url) logic to post-save (Title/Page) signals and compute them there, and store them in Title.path.
This means Page.get_absolute_url() should be A LOT faster now (=faster menus?), however someone better check that first.
It also means that page/title writes (create,update,move,blah) will be SLOWER, but in my opinion, optimizing reads over writes is preferable.
As a little side effect this commit also potentially fixes #493 due to also fixing #770.
And a further side effect, it will HIDE but NOT FIX #760. Potentially making that bug even more obscure (however it might still show up in the signals).
As you can see, I've changed a lot of tests (because they looked broken in my opinion), so this patch should get extra care.