Herald - add field + condition for Diffusion Commits for "On autoclose branch"

Summary:
Fixes T1461.

Adds

• FIELD_ALWAYS - now you could add this to a content type to always get notified
• FIELD_REPOSITORY_AUTOCLOSE_BRANCH - solves T1461
• CONDITION_UNCONDITIONALLY - used by these two fields to not show any value for the user to select

Test Plan: made a herald rule where diffs on autoclose branches would get flagged blue. made a diff on an autoclose branch and committed it. commit was flagged!

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Maniphest Tasks: T1461

Differential Revision: https://secure.phabricator.com/D7210