Simplify deployment - remove nginx service

[MDeLuise]

This PR simplify the deployment removing the nginx reverse proxy service.
This leads to the following improvements:

• less service used, more lightweight deployment
• possibility of changing backend and frontend services' port binding indipendently
• possibility of more granular CORS filtering in the backend

Also in this PR:

• renames FRONTEND_URL parameter (backend.env) to a more clear ALLOWED_ORIGINS
• updates the main README of the app
• set a fixed version for redis and mysql services
• fix requirement java version in the README
• add a check for backend available in the authentication window
• better error rendering in the authentication window