✨ 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}}/backend/app/Pipfile

@@ -20,7 +20,7 @@ pyjwt = "*"
 python-multipart = "*"
 email-validator = "*"
 requests = "*"
-celery = "~=4.3"
+celery = "*"
 passlib = {extras = ["bcrypt"],version = "*"}
 tenacity = "*"
 pydantic = "*"