Refactor backend to properly get user models and query models #193
Description
Basic Information
[Give a brief summary of what the issue is about]
Currently it is inconsistent across the apps on how the user models are being acquired and how models are being queried and consumed.
E.g. instead of using filters to select objects, use get_model_or_404
ensure all get_user_model is used everywhere
Other Information
[Are there any additional information that we should know about?]