Add a "Rule Detail" link to Herald email

Summary:
See PHI285. Ref T13130. After recent changes Herald sends email about rules, but the mail doesn't currently actually include a link to the rule.

Include a link for consistency and ease-of-use.

Test Plan: Edited a rule, looked at the resulting mail, saw a link to the rule.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13130

Differential Revision: https://secure.phabricator.com/D19413