feat: add support for python-dotenv to store secret key

[jxdv]

Closes #196.

Current workflow:

• if the .env file doesn't exists then we create it
• if FLASK_SECRET_KEY is already set and we didn't specify --flask-secret-key then don't touch it
• if FLASK_SECRET_KEY isn't set and --flask-secret-key isn't set, then use os.urandom to create cryptographically secure key
• if FLASK_SECRET_KEY is set, but we specified --flask-secret-key then we overwrite it

New changes:

• added new instructions to README
• increase size of access token to 12, to make it more secure
• remove junk from .gitignore, which we never use
• added python-dotenv dependency to poetry file and also refresh lock file

[orhun]

🚀