[Zurich] Add JS validation for external body field
[Zurich] Add JS validation for external body field

Ensures that an external body has been selected if the field is visible.

See mysociety/FixMyStreet-Commercial#718

appstud/fixmystreet,appstud/fixmystreet,nditech/fixmystreet,dracos/fixmystreet,appstud/fixmystreet,rbgdigital/fixmystreet,altinukshini/lokalizo,nditech/fixmystreet,appstud/fixmystreet,rbgdigital/fixmystreet,mhalden/fixmystreet,nditech/fixmystreet,dracos/fixmystreet,altinukshini/lokalizo,rbgdigital/fixmystreet,dracos/fixmystreet,nditech/fixmystreet,altinukshini/lokalizo,altinukshini/lokalizo,appstud/fixmystreet,dracos/fixmystreet,nditech/fixmystreet,rbgdigital/fixmystreet,nditech/fixmystreet,appstud/fixmystreet,appstud/fixmystreet,altinukshini/lokalizo,dracos/fixmystreet,mhalden/fixmystreet,altinukshini/lokalizo,mhalden/fixmystreet,dracos/fixmystreet,nditech/fixmystreet,mhalden/fixmystreet,mhalden/fixmystreet,rbgdigital/fixmystreet,mhalden/fixmystreet,rbgdigital/fixmystreet