Use specialist_sectors in RegisterableEdition

Previously it was looking through mainstream categories to find oil and
gas tags.

Also mixin Edition::SpecialistSectors into DetailedGuide.
Fix SpecialistSector validation for new editions

If you assign specialist_sector_tags to a new (unpersisted) edition they
won't get an edition ID and so won't be valid. This change makes them
check for the presence of edition instead and manually assigns it in
specialist_sector_tags=