Return first assigned category as parent for top-level page.

This should resolve gma/nesta#147.
I've just been looking at what they do, and I can't see a particularly strong reason to sort them alphabetically at all.

Perhaps it'd be better to just list the categories in whatever order the user puts them in the metadata. If they want them sorted alphabetically they can do it, otherwise it might make sense to list the most closely associated categories first.

Apologies for being indecisive on this, but it looks like it might be an opportunity to simplify. What do you think?