Support enabling a formal points field in Maniphest

Summary:
Ref T4427.

• New config option for labels, enabling, etc., but no UI/niceness yet.
• When enabled, add a field.
• Allow nonnegative values, including fractional values.
• EditEngine is nice and Conduit / actions basically just work with a tiny bit of extra support code.

Test Plan:

• Edited points via "Edit".
• Edited points via Conduit.
• Edited points via stacked actions.
• Tried to set "zebra" points.
• Tried to set -1 points.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4427

Differential Revision: https://secure.phabricator.com/D15220