Get mailman user using email if cached user_id is invalid.
When updating Mailman Core API from 3.0 to 3.1, some Django user's whoose user_ids are cached can have issues if the cached user_id is integer since the 3.1 API expects UUIDs.
This change makes it such that we lookup the user_id via email again if the cached user_id return 404.
Closes #35 (closed)
Edited by Abhilash Raj