Popup calendar for board term date inputs

In the interest of time, use the jQuery UI-based solution shipped
as part of the Date project, in the ``date_popup`` module.

In the future, it might be preferable to use a solution based on the
HTML5 date input (see
http://www.w3.org/TR/html5/forms.html#date-state-(type=date)).

Prior work on using the HTML5 date input with Drupal can be found
at:

* https://drupal.org/sandbox/davereid/1431984
* https://groups.drupal.org/node/252878

Addresses #3483