sectionpage entity urls

While this does not necessarily fix #605 it should make the situation much better.

I hope I did not break any links. I did test some but certainly not all.

**EDIT**: After feedback from @nidico I changed the whole thing. I also rebased against the current develop. For example this now includes some tests (with poor coverage but still better) and fixes everything noted in [@nidico's comment](https://github.com/xi/adhocracy/commit/b7ee83b050ce8df9c077be60a200ca7c7ffffe01#commitcomment-4479562). I would even be so bold to say this fixes #605.