fix: support multiple SSHPubKey records per user

There was one blazing issue with the previous implementation regardless
of the multiple records: we were generating fingerprints by storing
the key into a file and reading it with ssh-keygen. This is absolutely
terrible and was not meant to be left around (it was forgotten, my bad).

Took this opportunity to clean up a few things:
- simplify pubkey validation
- centralize things a bit better

Signed-off-by: Kevin Morris <kevr@0cost.org>

templates/partials/account_form.html

@@ -262,7 +262,7 @@
 
             <!-- Only set PK auto-fill when we've got a NewAccount form. -->
             <textarea id="id_ssh" name="PK"
-                      rows="5" cols="30">{{ ssh_pk }}</textarea>
+                      rows="5" cols="30">{{ ssh_pks | join("\n") }}</textarea>
         </p>
     </fieldset>