Refactor freeform field specification

Summary:
I want to attach the task also to revision, not only to commit by mentioning it in summary.
This is a preparation for it, useful by itself:

• One query instead of N.
• Lower CRAP score, yay!

Refs T945.

Test Plan:
My Test Plan is really plan this time.
I want to update Phabricator in the minute when I commit this.

Reviewers: 20after4, epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T945

Differential Revision: https://secure.phabricator.com/D3873