Relax validation primary_mainstream_category

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.
Merge pull request #1312 from alphagov/remove-mainstream-category-presence-validation

Relax validation primary_mainstream_category