Validate authorization config.

[datamel]

Follows on from: https://github.com/cylc/cylc-uiserver/pull/204 Validating of authorization config items c.CylcUIServer.user_authorization and c.CylcUIServer.site_authorization is limited to checking they are valid dictionaries. It would be helpful for users to get an error if they enter incorrect mutations, or have incorrect format in terms of nesting of dictionaries.