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>

test/test_adduser.py

@@ -53,4 +53,4 @@ def test_adduser_ssh_pk():
               "--ssh-pubkey", TEST_SSH_PUBKEY])
     test = db.query(User).filter(User.Username == "test").first()
     assert test is not None
-    assert TEST_SSH_PUBKEY.startswith(test.ssh_pub_key.PubKey)
+    assert TEST_SSH_PUBKEY.startswith(test.ssh_pub_keys.first().PubKey)