Allow users to drop .ics files on calendar views to import them

Summary:
Ref T10747. When a user drops a ".ics" file or a bunch of ".ics" files into a calendar view, import the events.

(Possibly we should just do this if you drop ".ics" files into any application, but we can look at that later.)

Test Plan: Dropped some .ics files into calendar views, got imports.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16722