Standardize SSH key storage

Summary:
Ref T5833. This fixes a few weird things with this table:

• A bunch of columns were nullable for no reason.
• We stored an MD5 hash of the key (unusual) but never used it and callers were responsible for manually populating it.
• We didn't perform known-key-text lookups by using an index.

Test Plan:

• Ran migrations.
• Faked duplicate keys, saw them clean up correctly.
• Added new keys.
• Generated new keys.
• Used bin/auth-ssh and bin/auth-ssh-key.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5833

Differential Revision: https://secure.phabricator.com/D10805