Prepare for event imports in Calendar

Summary:
Ref T10747. Adds a bunch of stuff so we can keep track of which events we've imported from external sources.

This doesn't do anything yet: you can't actually import anything.

Test Plan:

• Ran bin/storage upgrade.
• Clicked "Imports", saw an empty wasteland.
• Created/edited events.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16696