Use appropriate Ajax URI creation mechanism

[khituras]

We had issues with CORS issues when using a reverse proxy. The quick and dirty fix was to remove the port from the URI (because the CORS restrictions got triggered when the Ajax URLs contained the port number when the original URL did not).
Instead, use the boolean secure parameter in link.toAbsoluteUri(boolean) and set it to the value of the PRODUCTION_MODE symbol.

WARN: This means that PRODUCTION_MODE must be set to false in AppModule for local developments (as it should have been anyway) and to true when deploying to the server (with the reverse proxy).