container pages

This adds a new type (function) of pages: Container pages.

Container pages can be used as containers for other pages. They basically have a title and a description. The view also contains a list of subpages in a prominent position.

I did disable the options `sectionpage`, `allow_comment`, `allow_selection` and `formatting` for container pages. The last one is always true. The others are just not relevant in this case.

I did not disable anything else (categories, tags, ...) because I figured they might still be useful.

I did also not hide the subpages of containers from the page index. I did not know if that was wanted.