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

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

BaseDropdownField and providing field_id, label, and options 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 dropdown field and verified that the

  state was correctly rendered when loading the page and correctly stored

  when the dropdown was changed.