exclude sectionpages from select_page

One of the bugs still open from #535 is that "sectionpages can be selected from proposals". I noticed that in fact it does not make sense to select a sectionpage any where `select_page` is used.

This might be slow because it uses a lot of `Page.is_sectionpage()`. I am not completely sure but I guess that one is slow because it is recursively called on a pages parents. We should at least think about caching it.