CodeChecker authentication fixed

If the super_user field was missing from the
config file, CodeChecker authentication failed
for all users.

web/server/codechecker_server/session_manager.py

@@ -539,6 +539,9 @@ def get_db_auth_session_tokens(self, user_name):
 
     def __is_root_user(self, user_name):
         """ Return True if the given user has system permissions. """
+        if 'super_user' not in self.__auth_config:
+            return False
+
         if self.__auth_config['super_user'] == user_name:
             return True