Make .ics export less scary and attach .ics files to event mail

Summary:
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.

Test Plan:

• 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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16559