Use `hash` to compare XForms and Assets
Use `hash` to compare XForms and Assets

Fall back on `date_modified` only when no `hash` is available.
Fixes #661.

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