When summing points on a workboard, display sum with same precision as most-precise value

Summary:
Fixes T11703. This mostly avoids rounding errors.

If point values include "0.001", we also get three digits of precision: 1.000.

Maybe useful if your point field is called "bitcoins" or something.

Test Plan:
Before:

After:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11703

Differential Revision: https://secure.phabricator.com/D16601