Fix time dropdown overlapping the input field when list is open on top of the input field

Summary: As in trello card from kitt

Test Plan:
Currently used in coach recommendations. Create
recommendation, add due date and change time see that the list of
available times does not overlap input field.

Reviewers: dylan, khirasaki

Reviewed By: dylan

Differential Revision: http://phabricator.khanacademy.org/D3555