Allow Calendar exports to be disabled

Summary:
Ref T10747. This adds disable/enable to exports.

Mostly useful if you leak a URI by accident.

Test Plan:

• Disabled and enabled exports.
• Verified that disabled exports don't actually export any data.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16681