Separate SSH key management from the settings panel

Summary:
Ref T5833. I want to add SSH keys to Almanac devices, but the edit workflows for them are currently bound tightly to users.

Instead, decouple key management from users and the settings panel.

Test Plan:

• Uploaded, generated, edited and deleted SSH keys.
• Hit missing name, missing key, bad key format, duplicate key errors.
• Edited/generated/deleted/etc keys for a bot user as an administrator.
• Got HiSec'd on everything.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5833

Differential Revision: https://secure.phabricator.com/D10824