feature (app): add app config

[subotic]

Summary

• add application config at runtime
• move constants to app.constants.ts
• document basic configuration

Issues

• resolves Editable Config Files #259

[subotic]

@kilchenmann this works now but only without AgularFireModule. To be able to work with the AngularFireModule, we need to provide the AngularFireModule config by using DI as provided in 5.0.0-rc.9 (needs angular 6): https://github.com/angular/angularfire2/releases/tag/5.0.0-rc.9. Is there a timetable regarding the upgrade to Angular 6?

[kilchenmann]

The AngularFireModule is needed by the Beol beta release.
We're still working on the schedule. The whole process of updating Salsah to Angular 6 depends on the new core module which includes the services. I hope this is done by the end of June.

[subotic]

The AngularFireModule is needed by the Beol beta release.

Yes, I know. That's why this cannot be merged and Angular 6 is needed.