This change adds Python and JavaScript classes to add date-type fields in

review requests. Extensions can use this very simply by just subclassing

BaseDropdownField and providing field_id and label attributes. The

resulting value will be stored in extra_data as expected.

Ran js-tests.
Ran unit tests.
Created a test extension that used a date field and verified that the

  state was correctly rendered when loading the page and correctly stored when

  the date was changed. Verified that invalid dates were cleared out.