Indicate buttons disabled in JX.Workflow

Summary:
We are marking disabled buttons with 'disabled' class in behavior-form.
But we ignore JX.Workflow there because it has its own handling.
But this handling doesn't set class so the button is disabled but it is not indicated to user.

It causes troubles in Clowncopterize where users report that browser freezes before doing anything after clicking it. It probably happens also on other places.

This diff solves it by using CSS3 selector on attribute (contrary to explicitly setting class in JX.Workflow).

Test Plan:
Add sleep(3) to DifferentialCommentSaveController.
Clowncopterize empty comment.

Reviewers: epriestley, tuomaspelkonen

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2264