Exclude Enketo `preview_iframe_url`
Exclude Enketo `preview_iframe_url`

See https://www.flowdock.com/app/kobotoolbox/ux-ui/threads/Uq7EaesWWh_m7qmzsYeyzp18XN0

Conflicts:
	kpi/deployment_backends/mock_backend.py

kobotoolbox/kpi,kobotoolbox/kpi,kobotoolbox/kpi,onaio/kpi,onaio/kpi,onaio/kpi,onaio/kpi,kobotoolbox/kpi,kobotoolbox/kpi