Fix some minor Calendar issues, including a paging issue on imports

Summary:
Fixes T11808. I couldn't reproduce the issue there locally so I'm just cheating a little bit until a better reproduction case shows up.

We don't need to do a full load here anyway, and testing for any row is more efficient.

Test Plan: Poked around imports without issues, but couldn't reproduce this problem locally.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11808

Differential Revision: https://secure.phabricator.com/D16787