sectionpage

This implements sectionpages (#535).

It adds four new options to pages: `sectionpage`, `allow_comment`, `allow_selection` and `always_show_original`. It is obvious what the first one should do. However, I had problems with the others. I tried to apply `allow_comment` and `allow_selection` but I am neither sure if I found everything nor if I have interpreted the scope of these options as intended.

`always_show_original` has currently no effect at all (also see line 252 in src/adhocracy/model/delegateable.py). That is mainly because I did not understand what the alternative to showing `HEAD` would be. Additionally I was not sure how to handle the preexisting `formatting` option. I don't think it makes any difference in the UI.

@Reichertson would like to show the sectionpages to a designer to make some adjustments. So it would be great if we could deploy this on a test server as soon as possible.

The new interface makes heavy use of iframe overlays (#536) but also sufferes from #543. I think it worked out well: It looks fancy because of the animated overlay resizing, the core features work well but there still is a lot of room for improvements.

I am looking forward to seeing this in action. I would really like to do some user testing or something to see if it works as expected and where we need to change things. However I need to work on something else next week so I also won't be available to work on this PR.