R1:fddb506e987f

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…