Set an explicit default value for the bulk action control

Summary:
Ref T13025. See https://discourse.phabricator-community.org/t/bulk-edit-no-actions-available/1011/1.

I'm not sure if this is what the user is seeing, but in Chrome, the <select /> does not automatically get set to the first valid value like it does in Safari.

Set it to the first valid value explicitly.

Test Plan: In Chrome, bulk editor previously hit a JS error when trying to read a bad action off the <select />. After patch, bulk edits go through cleanly.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13025

Differential Revision: https://secure.phabricator.com/D18923