Add email preference links to email footers

Summary: Ref T1217, Add link to email preferences to email template

Test Plan: Add comment to object like Maniphest task, check that email has a footer with a link to email preferences.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T1217

Differential Revision: https://secure.phabricator.com/D10883