Return top-priority category as parent for top-level page.

This should resolve gma/nesta#147.
Checking the priority isn't what we want here. The numerical suffix on the end of the category metadata permalinks (e.g. "foo:1") isn't there to say which of the categories is the most important to this page, it's there to define the sort order of the pages listed on the category page (i.e. on the `/foo` page).

All we want for the `#parent` method is the first category in the list. So you'll be able to return `categories[0]` after splitting the existing `categories` method into one that sorts and one that doesn't.

I've not looked through the templates to work out where we ought to be calling `sorted_categories`, but don't hesitate to ask if you're unsure about any of them…