When administrators revoke SSH keys, don't include a "security warning" in the mail

Summary:
Depends on D18906. Ref T13043. When SSH keys are edited, we normally include a warning that if you don't recognize the activity you might have problems in the mail body.

Currently, this warning is also shown for revocations with bin/auth revoke --type ssh. However, these revocations are safe (revocations are generally not dangerous anyway) and almost certainly legitimate and administrative, so don't warn users about them.

Test Plan:

• Created and revoked a key.
• Creation mail still had warning; revocation mail no longer did.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13043

Differential Revision: https://secure.phabricator.com/D18907