Add support for a "bool" type to Maniphest's default field extensions

Summary:

• On the edit view, this is represented as a checkbox.
• On the detail view, it renders with a user-selectable string.

Test Plan: Added a bool field to my local install, checked and unchecked it.

Reviewers: zeeg, jungejason, btrahan

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: https://secure.phabricator.com/D1277