✨ 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/scripts/lint.sh

@@ -5,4 +5,4 @@ set -x
 autoflake --remove-all-unused-imports --recursive --remove-unused-variables --in-place app --exclude=__init__.py
 isort --multi-line=3 --trailing-comma --force-grid-wrap=0 --combine-as --line-width 88 --recursive --apply app
 black app
-vulture app
+vulture app --min-confidence 70