Send forced mail on SSH key edits

Summary:
Ref T10917. This cheats fairly heavily to generate SSH key mail:

• Generate normal transaction mail.
• Force it to go to the user.
• Use setForceDelivery() to force it to actually be delivered.
• Add some warning language to the mail body.

This doesn't move us much closer to Glorious Infrastructure for this whole class of events, but should do what it needs to for now and doesn't really require anything sketchy.

Test Plan: Created and edited SSH keys, got security notice mail.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10917

Differential Revision: https://secure.phabricator.com/D15948