Add a description preview to maniphest create / edit panel

Summary: also fixes a small bug where the page title was always "Create Task".
switch it to the header name which is much more descriptive / correct IMO.

Test Plan:
created a new task and watched the description preview update.
edited an old task and saw the description preview populate with the correct
existing data.
edited an old task and edited the description and saw the description preview
update

Reviewers: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1489