Make more of the Calendar export workflow work

Summary:
Ref T10747.

• Adds a "Use Results..." dropdown to query result pages, with actions you can take with search results (today: create export; in future: bulk edit, export as excel, make dashboard panel, etc).
• Allows you to create an export against a query key.
  • I'm just using a text edit field for this for now.
• Fleshes out export modes. I plan to support: public (as though you were logged out), privileged (as though you were logged in) and availability (event times, but not details).

This does not actually export stuff yet.

Test Plan: Created some exports. Viewed and listed exports.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16676