Replace ember-cli-mirage with ember-mirage. This new, much more embroider compatible, addon is the way forward for mirage support.

This isn't needed anymore as ember-mirage doesn't run in development by
default.

With ember-mirage we lose a lot of the automagic configuration and detection of our factories and models as well as auto calling the `mirage/config` to setup the server. Instead we can just export all this information in common and import it explicitly into our config in each app.

We now provide the mirage server creation explicitly in our tests so we
need to call it from a different place.

I'm not sure why I grabbed the ember data import, but I think relying on
the standard is better for us.

This import requires us to be using ember-cli-mirage and we aren't
anymore. Plus we were barely using it here and can just as easily check
these values ourselves.