Changes required for Heroku deployment

[blms]

In this PR

Various changes for Heroku deployment:

• Remove .python-version from .gitignore and set to the supported 3.11
• Only use django_browser_reload in debug mode (i.e. local dev env)
• Ensure all static assets have dynamic paths (using template tags)
• Use WhiteNoise middleware to serve static assets in production
• Use AWS S3 to serve media content in production
• Use Gunicorn to run the server in production
• Allow the use of a single DATABASE_URL environment variable to configure the db, via dj-database-url
• Add Procfile for Heroku and Node scripts to aid deployment
  • Move Tailwind Node devDependencies to dependencies, and ensure the Tailwind package (theme/static_src/) is installed and built