Allow InvitedUser to be initialised with folder_permissions

The `folder_permissions` property has no effect and is not used yet, but
it needs to be added before we add a `folder_permissions` column to the
`InvitedUser` model in notifications-api. This is because we initialize
the InvitedUser class with the response from notifications-api.

app/models/user.py

@@ -184,7 +184,16 @@ class User(UserMixin):
 
 class InvitedUser(object):
 
-    def __init__(self, id, service, from_user, email_address, permissions, status, created_at, auth_type):
+    def __init__(self,
+                 id,
+                 service,
+                 from_user,
+                 email_address,
+                 permissions,
+                 status,
+                 created_at,
+                 auth_type,
+                 folder_permissions=None):
         self.id = id
         self.service = str(service)
         self.from_user = from_user