Made static files for backend configurable

The changes in the MR here are mostly how the static files are handled.

Basically the configuration is in a way to handle all static files mounted to the nginx container under /static/, so in case for the backend files, they are accessable via 'https://localhost/static/backend/css/swagger-ui.css` for example.

This config has the benefit that we can just add another mount to the container (for example to the static files from the cv) and make them accessible right away without any further change on the nginx configuration.

Nevertheless, the url that flask uses to access those static files is now configurable with an env variable (in case you need the /backend/static instead - just change the env variable and check both the volume mounts and the rules for the static files in the nginx).

I also tried to make the adjustments that I see already as necessary for the gfz deployment. I haven't tested those, so there can be some further issues.