Relax validation primary_mainstream_category

https://www.pivotaltracker.com/story/show/67200884

It should only be required if one or more other_mainstream_categories
have been selected. This allows detailed guides to be added to
specialist sectors without having a mainstream category.

Mainstream categories will eventually be phased out in favour of
specialist sectors, at which point this whole thing will disappear.

(This is a new copy of #1311 due to Github weirdness)