feat: allow different base path for fastboot

[szymsza]

When deployed using redpencil/fastboot-app-server, the app needs to make requests to /sessions when running in the browser and http://backend/sessions when running in Fastboot. This was not possible, which led to either the browser or the Fastboot side not working.

This PR adds a new configuration option fastbootBasePath. If present, this value is used as the base path in Fastboot and basePath is used in the browser. If absent, basePath is used for both environments (as was the case until now).

To make the deployment work, this piece of code should be added to config/environment.js:

  if (environment === 'production') {
    ENV.fastbootBasePath = 'http://backend/sessions';
  }