cas,middlebury_cas: Updated the CAS module. Proxy chain config is now in the cas module.

I had added an 'allowed_proxy_chains' setting to the middlebury_cas module, but this
configuration is now supported by the cas module, so we don't need our own version.
cas_attributes,middlebury_cas_settings: Upgraded the cas_attributes and updated the settings feature to match.

Upgrade process: Apply database upgrade, then revert the feature if needed.