R1:0b5abf7bb51a

Allow "0" to be a valid workboard column point limit

Summary:
Fixes T6580. Now:

- Empty field means "unlimited".
- Zero means 0.
- Nonzero means that number.

(Although you can now have fractional points, I didn't change columns to allow fractional limits, because too bad.)

Test Plan: {F1103688}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6580

Differential Revision: https://secure.phabricator.com/D15239