✨ Add Items (crud, models, endpoints), utils, refactor (#14)

* Update CRUD utils to use types better.
* Simplify Pydantic model names, from `UserInCreate` to `UserCreate`, etc.
* Upgrade packages.
* Add new generic "Items" models, crud utils, endpoints, and tests. To facilitate re-using them to create new functionality. As they are simple and generic (not like Users), it's easier to copy-paste and adapt them to each use case.
* Update endpoints/*path operations* to simplify code and use new utilities, prefix and tags in `include_router`.
* Update testing utils.
* Update linting rules, relax vulture to reduce false positives.
* Update migrations to include new Items.
* Update project README.md with tips about how to start with backend.

{{cookiecutter.project_slug}}/README.md

@@ -53,7 +53,9 @@ If your Docker is not running in `localhost` (the URLs above wouldn't work) chec
 
 ### General workflow
 
-Add and modify SQLAlchemy models in `./backend/app/app/db_models/`, Pydantic models in `./backend/app/app/models` and API endpoints in `./backend/app/app/api/`.
+Open your editor at `./backend/app/` (instead of the project root: `./`), so that you see an `./app/` directory with your code inside. That way, your editor will be able to find all the imports, etc.
+
+Modify or add SQLAlchemy models in `./backend/app/app/db_models/`, Pydantic models in `./backend/app/app/models/`, API endpoints in `./backend/app/app/api/`, CRUD (Create, Read, Update, Delete) utils in `./backend/app/app/crud/`. The easiest might be to copy the ones for Items (models, endpoints, and CRUD utils) and update them to your needs.
 
 Add and modify tasks to the Celery worker in `./backend/app/app/worker.py`.