Use shared nginx code

Use shared Nginx code.

• Removed all Nginx related code except "nginx.conf" which contains backend-specific Nginx settings
• Nginx Docker image of the auth-backend is now in use
• Most of the documentation related to Nginx has now been moved to auth-backend repository

How to test:

• Run all locally with Docker compose
  docker-compose up -d --build
• Run health check (for Nginx)
  curl -kL https://0.0.0.0:4443/health_check
  or
  curl -kL https://localhost:4443/health_check
• Run health check (for the BE itself)
  curl -kL https://0.0.0.0:4443/healthcheck
  or
  curl -kL https://localhost:4443/healthcheck
• To check if the if the default nginx.conf file has been replaced with one in this repository run:
  docker exec -it nginx-presets-backend cat nginx.conf
• Test that everything else works as before

Extra:

• Run everything with Docker (without Docker Compose and SSL) according to the readme here

Closes #44 (closed)