* Add step reordering service endpoint to API [SCI-6891] * Generalize reorder validation [SCI-6891] * Add endpoint for reordering step elements, fix issues [SCI-6892] * Add appropriate serializers [SCI-6891][SCI-6892] * Add step elements to step serializer [SCI-6891] * Simplify routes, add locking [SCI-6891]