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

scripts/ssh/ssh-auth.php

@@ -34,9 +34,15 @@ foreach ($keys as $ssh_key) {
 
   $type = $ssh_key->getKeyType();
   $type = preg_replace('@[\x00-\x20]+@', '', $type);
+  if (!strlen($type)) {
+    continue;
+  }
 
   $key = $ssh_key->getKeyBody();
   $key = preg_replace('@[\x00-\x20]+@', '', $key);
+  if (!strlen($key)) {
+    continue;
+  }
 
   $options = array(
     'command="'.$cmd.'"',