Stop moving secondary to primary specialist sector from falling over

If a user attempted to move a "secondary" tag to the "primary" tag, an exception would be raised because of the uniqueness constraint on the `SpecialistSector` model.

See: https://errbit.production.alphagov.co.uk/apps/53020d6c0da11585f10000e7/problems/535a240a0da1151271000086
Yep, as explained in https://github.com/alphagov/whitehall/commit/b6b492434851ecf29b5b191be960c12d136cb1dd