Support disabling calendar imports

Summary: Ref T10747. This doesn't do much for ICS file imports (you can't disable them since it doesn't do anything meaningful) but will matter more for ICS-subscription imports later.

Test Plan: Clicked "Disable" on an ICS file import, got explanatory dialog.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16702