remote_billing: Sort out remote_billing_identities typing.

This does two important things:
1. Fix return type of get_identity_dict_from_session to correctly be
   Optional[Union[RemoteBillingIdentityDict, LegacyServerIdentityDict]].
   RemoteBillingIdentityDict is the type in the 8.0+ auth flow,
   LegacyServerIdentityDict is the type in old servers flow, where only
   the server uuid info is available.
2. The uuid key used in request.session["remote_billing_identities"]
   should be explicitly namespaced depending on which flow and type
   we're
   dealing with - to avoid confusion in case of collisions between a
   realm and server that have the same UUID. Such a situation should not
   occur naturally and I haven't come up with any actual exploitation
   ideas that could utilize this by manipulating your server/realm
   uuids, but it's much easier to just not think about such collision
   security implications by making them impossible.

corporate/lib/decorator.py

@@ -49,14 +49,11 @@ def authenticated_remote_realm_management_endpoint(
             return render(request, "404.html", status=404)
 
         realm_uuid = kwargs.get("realm_uuid")
-        server_uuid = kwargs.get("server_uuid")
         if realm_uuid is not None and not isinstance(realm_uuid, str):
             raise TypeError("realm_uuid must be a string or None")
-        if server_uuid is not None and not isinstance(server_uuid, str):
-            raise TypeError("server_uuid must be a string or None")
-        remote_realm = get_remote_realm_from_session(
-            request, realm_uuid=realm_uuid, server_uuid=server_uuid
-        )
+
+        remote_realm = get_remote_realm_from_session(request, realm_uuid)
+
         billing_session = RemoteRealmBillingSession(remote_realm)
         return view_func(request, billing_session)