♻️ Refactor backend, settings, DB sessions, types, configs, plugins (#158)

* ♻️ Refactor backend, update DB session handling

* ✨ Add mypy config and plugins

* ➕ Use Python-jose instead of PyJWT

as it has some extra functionalities and features

* ✨ Add/update scripts for test, lint, format

* 🔧 Update lint and format configs

* 🎨 Update import format, comments, and types

* 🎨 Add types to config

* ✨ Add types for all the code, and small fixes

* 🎨 Use global imports to simplify exploring with Jupyter

* ♻️ Import schemas and models, instead of each class

* 🚚 Rename db_session to db for simplicity

* 📌 Update dependencies installation for testing

{{cookiecutter.project_slug}}/backend/app/.flake8

@@ -0,0 +1,3 @@
+[flake8]
+max-line-length = 88
+exclude = .git,__pycache__,__init__.py,.mypy_cache,.pytest_cache