Ref T10747.

• Remove the warning dialog since these files don't seem to do anything confusing/problematic in Calendar.app or Google Calendar. Those importers generally need to be defensive about how they handle random ".ics" files from arbitrary third parties anyway, and this makes testing imports easier since we have a GET-table ".ics" URI for public events.
• Attach ".ics" files to email.

• Clicked "Export as .ics", got an ICS file.
• Used "bin/mail show-outbound" to review an ICS attachment, although I don't actually have real mail set up locally so this may still be a little funky.