Pull out JWT auth validation into validator class

Also fixes a small bug in validation (yay tests!)

test/test_external_jwt_authn.py

@@ -36,7 +36,8 @@ def fake_jwt(requires_email=True):
   getuser_url = server_url + '/user/get'
 
   jwt_auth = ExternalJWTAuthN(verify_url, query_url, getuser_url, 'authy', '',
-                              app.config['HTTPCLIENT'], 300, public_key.name,
+                              app.config['HTTPCLIENT'], 300,
+                              public_key_path=public_key.name,
                               requires_email=requires_email)
 
   with liveserver_app(jwt_app, port):