Move typehaead dropdown up a few z-index levels

Summary: Fixes T5451.

Test Plan:
  - Added a custom date field, observed icon render underneath typeahead results.
  - Couldn't think of any reasons that typeaheads should be under any of the elements with index 9, 10, 11, or 12.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5451

Differential Revision: https://secure.phabricator.com/D9718