Move SSHKey table to Auth database

Summary: Ref T5833. Since these will no longer be bound specifically to users, bring them to a more central location.

Test Plan:
  - Edited SSH keys.
  - Ran `bin/ssh-auth` and `bin/ssh-auth-key`.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5833

Differential Revision: https://secure.phabricator.com/D10791