Allow "Test Plan" to be disabled in config

Summary:
This is a somewhat common request, and far more difficult than necessary currently.

I think the field is useful enough to leave it default-enabled, but there's wide diversity in testing philosophy.

Test Plan: Verified "test plan" field appeared. Disabled config. Verified "test plan" field vanished.

Reviewers: btrahan, vrana, jungejason

Reviewed By: vrana

CC: aran, asouza

Differential Revision: https://secure.phabricator.com/D2193