web: Allow CASProvider config to be any source

By defining ``env`` in ``CASProvider``, the superclass implementation
of ``is_configured`` requires the variables to be set in the actual
environment.  Configuration can come from reading in ``/run/secrets``,
an ``.env`` file, or the environment.

This commit defines our own ``is_configured`` that checks ``CONFIG``
for the needed variables instead of relying on the superclass.

Ref: AP-412

Author: awilfox

willa/web/cas_provider.py

@@ -66,6 +66,9 @@ def __init__(self) -> None:
             'scope': 'openid profile berkeley_edu_groups',
         }
 
+    def is_configured(self) -> bool:
+        return all(var in CONFIG for var in self.env)
+
     async def get_token(self, code: str, url: str) -> str:
         request = {
             'client_id': self.client_id,