Add a "Subtype" tag to the task graph view in Maniphest

Summary: See PHI1466. When an install defines task subtypes, show them on the task graph.

Test Plan:

• On desktop with subtypes defined, column is visible.
• On desktop with subtypes not defined, column is hidden.
• On mobile, column is hidden.

{F6896845}

Differential Revision: https://secure.phabricator.com/D20842