Make a service model and use for permissions

Having the service floating about as JSON is a bit flakey. Could easily
introduce a mistake where you mistype the name of a key and silently
get `None`.

Also means doing awkward things like `if 'permission' in
current_service['permissions']`, whereas for users we can do the
much cleaner `user.has_permission()`.

So this commit:
- introduces a model
- adds a `.has_permission` method similar to the one we have for users

app/notify_client/models.py

@@ -261,3 +261,13 @@ class AnonymousUser(AnonymousUserMixin):
     # set the anonymous user so that if a new browser hits us we don't error http://stackoverflow.com/a/19275188
     def logged_in_elsewhere(self):
         return False
+
+
+class Service(dict):
+
+    def __init__(self, _dict):
+        # in the case of a bad request current service may be `None`
+        super().__init__(_dict or {})
+
+    def has_permission(self, permission):
+        return permission in self['permissions']