Fix two minor points UI issues

Summary:
Ref T4427.

• When points are configured, show them on the task detail page (just a simple property, at least for now).
• Typecast points better to avoid "joe changed points from 12 to 12." beacuse we compare the stored value (as a string) to the new value (as a double).

Test Plan:

• Saw points on detail view.
• Created task with points, then edited it without touching points. No more spurious "changed from 12 to 12" transaction.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4427

Differential Revision: https://secure.phabricator.com/D15223