R1:2ab07ed29bd9

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