Support "<select />" custom fields in bulk editor

Summary: Ref T13025. Fixes T5689. A straightforward change!

Test Plan: Used the bulk editor to modify a custom "select" field like the one in T5689.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13025, T5689

Differential Revision: https://secure.phabricator.com/D18879