Show task counts in column headers on Workboards

Summary: Ref T4427. This always counts 1 task = 1 point. The tricky bit is making this update in JS.

Test Plan: {F190900}

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4427

Differential Revision: https://secure.phabricator.com/D10275