Fixing z-index on calendar date badges.

Summary: Fixes T4787, decreased z-index on calendar badges so that they don't sit on top of notification dropdowns when dropdowns are expanded. Not sure why the badges had z-index 10, but please let me know if there was a more substantial reason for this.

Test Plan: If neither notification dropdowns have content, create enough messages to populate at least 5 rows, open calendar, expand messages dropdown, verify that underlying calendar date badges do not appear over the dropdown.

Reviewers: chad, Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4787

Differential Revision: https://secure.phabricator.com/D8770