[events] Initial support for iCloud and Outlook.com/Hotmail

Summary:
[events] Initial support for iCloud and Outlook.com/Hotmail
Also makes the Events / Contacts interface more generic.

Fixes T306

Test Plan: Add events on calendars through web interface, see them show up from the API

Reviewers: emfree

Reviewed By: emfree

Subscribers: mg

Maniphest Tasks: T306

Differential Revision: https://review.inboxapp.com/D345