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