Ability to import overlays (e.g public holidays) in to the Gantt days as faint vertical bars Merged

We would like to show public holidays and office closures on the Releases Gantt chart as a faint vertical bar similar to that shown in MS Project (screenshot). Ideally this would be implemented as a call to a remote REST API which returns iCal format data. An alternative may be to manually upload a file containing this data (iCal).