Release 6.20.1: Disable specific calendar features

[changerequest]

Some custom backend services for calendar cannot handle attachments or flags. Those feature can now be disabled via configuration.

New settings:
ui/global/disableFeature/calendar-attachments = true
ui/global/disableFeature/calendar-flags = true
ui/global/disableUpsell/calendar-attachments = true
ui/global/disableUpsell/calendar-flags = true