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
