Don't render the Maniphest edit form bottom-of-page preview panel if "Description" is locked or hidden

Summary:
See https://discourse.phabricator-community.org/t/hidden-description-field-in-maniphest-task-breaks-form/1432.

If you hide the "Description" field in Maniphest, we still try to render a remarkup preview for it. This causes a JS error and a nonfunctional element on the page.

Instead, hide the preview panel if the field has been locked or hidden.

Test Plan:

• Hid the field, loaded the form, no more preview panel / JS error.
• Used a normal form with the field visible, saw a normal preview.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19432