Changes

Fixes communication between frontend and backend
On the one hand, the frontend was always using the pre-production 
backend. On the other hand, the pre-production frontend is being served
over HTTPS, but the backend URL was using the HTTP protocol, so there
was a security error.
This commit fixes these problems, with the cost that now during 
development it is necessary to use Angular's own server for the frontend
(although this was already the recommended option).