Backend: Cleanup

[Topvennie]

Important

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1. Uniform error messages in the endpoints (Try GET, POST, PUT, PATCH on every endpoint (tedious I know))
2. Add exception handling
3. Go through TODO's
4. Go through all the translations
5. Add realistic fixture's / prepare a db for production
6. Uniform responses to endpoints (e.g. creating should return an object's details)
7. Startup tasks
8. Support PATCH where possible
9. Go through the models and look at foreign keys and their database on_delete policy

Nice to have

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1. Add comments
2. Uniform code structure
3. Model consistency (e.g. some have specified ID, some don't (also models in authentication and notifications))
4. Only import from packages, not relative (no ../../authentication/models)
5. Add implicit fields to all models
6. Use ID as primary keys, not name
7. Periodically file & db cleanup
8. Optimize amount of db calls
9. Generalise some permissions

Optional

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1. Add typings everywhere (django-typings, djangorestframework-typing)