R1:49448a87c17c

Rough in most of Calendar exports

Summary:
Ref T10747. Rough flow is:

- Run a query.
- Select a new "Export Events..." action.
- This lets you define an "Export", which has a unique URL you can paste into Google Calendar or Calendar.app or whatever.

Most of this does nothing yet but here's the boilerplate.

Test Plan: Doesn't do anything yet.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16675