Make workboard columns fixed-height and internally scrollable

Summary:
Ref T5240.

• Columns are fixed height.
• Columns scroll internally.
• Drag behaviors generally align with these column behaviors.

Test Plan: {F1099061}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5240

Differential Revision: https://secure.phabricator.com/D15209