Replace dependency on the Http module by new HttpClient module

[deeleman]

The HttpClient library was introduced back in Angular 4.3 as a replacement for the good old Http module which has been DEPRECATED as for Angular 5.

The HttpClient library also introduces a more elegant API for handling request headers and response objects and HTTP interceptors, which we will be wanting to use in the future.

[gashcrumb]

I think this is a good move to make and I think this is a perfectly fine time to do it... @kahboom @seanforyou23 thoughts?

[kahboom]

I'm all about this, and if we could simultaneously remove Restangular that would be splendid (sorry, author is really cool but we don't need it).