Add a "points" field to tasks

Summary:
Currently never read or written.

Supports fractions.

There's no such thing as an unsigned double so this also supports negative values, technically, although I'll eventually prevent this in the UI.

Test Plan: bin/storage upgrade, then created and edited a task. Nothing was different.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15218