add builtin two-factor auth support

The builtin two-factor authentication is based on time-based one time
passwords (RFC 6238) which works with Authy, Google Authenticator and
other compatible apps.

dataprovider/sqlcommon.go

@@ -806,6 +806,7 @@ func getAdminFromDbRow(row sqlScanner) (Admin, error) {
 		admin.Description = description.String
 	}
 
+	admin.SetEmptySecretsIfNil()
 	return admin, nil
 }