Change a way of handling a settings

[korycins]

• Drop simple settings object build based on Django solution
• Add pydantic Settings object to handle all app settings. More details can be found here: https://fastapi.tiangolo.com/advanced/settings/#settings-and-environment-variables

Tldr:

class AppSettings(Settings):
    project_dir = PROJECT_DIR
    static_dir = os.path.join(PROJECT_DIR, "static")
    templates_dir = os.path.join(PROJECT_DIR, "templates")

or

class AppSettings(Settings):
    class Config:
        env_file = ".env"