Link proxy column headers on workboards to proxied projects

Summary: Ref T10010. Allows you to click "Milestone 99" to jump directly to that project.

Test Plan:

• Clicked milestone header, went to milestone.
• Clicked normal column header, nothing happened. Wow!

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10010

Differential Revision: https://secure.phabricator.com/D15204