Check permission level on the user config files

[datamel]

Follows on from https://github.com/cylc/cylc-uiserver/pull/204

As mentioned in the authorization proposal, for security reasons, the user config.py file, which is located in ~/.cylc/hub/config.py, should not be writable by anyone other than the user (otherwise the authorization settings could be manipulated). A check should be made to ensure permissions for group and others are not writable and if not an error should be raised.