Allow encrypted mail to be more specific about which object is affected

Summary:
Depends on D19487. Ref T13151. See PHI647. For some objects, like revisions, we can build slightly more useful secure email without actually disclosing anything.

In the general case, the object monogram may disclose information (#acquire-competitor) but most do not, so applications can whitelist an acceptable nondisclosing subject and link.

Support doing this, and make Differential do it. When we don't have a whitelisted URI but do know the object the mail is about, include a generic PHID-based URI; these are always nondisclosing.

Test Plan:

• Without the Differential changes, sent normal mail (no changes) and secure mail (new generic PHID-based link).
• With the Differential changes, sent secure mail; got richer subject and body link.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13151

Differential Revision: https://secure.phabricator.com/D19488