Generate newValue for ManiphestTaskPointTransaction

Summary: I think this is the correct fix, sets a consistent value for transactions, old and new, for Maniphest point values.

Test Plan:
Edit title, see no point feed story, set points, see point story, set points to same value, see no story, remove points, see remove point story.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17885