Serve the Angular app from Spring boot

[wariuccio]

Addresses https://github.com/booklore-app/booklore/issues/1404

This PR updates the Spring Boot application to make it able to serve the pre-built files of the Angular app from a location on disk. This in turn enables us to get rid of Nginx since we only have a single web server (the Spring Boot application) able to serve everything we need.

I tested this with the example Docker compose file (substituting in a Docker image I built myself with the changes in this PR) and it seems to work. There are a couple things I am not too sure about, I'll leave comments inline.