Start using ember-cli-mirage factories

[Turbo87]

Instead of using fixtures it is recommended to use factories to setup acceptance tests. This PR introduces a first set of factories and uses them in a few of the existing tests to simplify them. This PR is not yet converting everything to factories, but can be merged at any point.

@samselikoff would be awesome if you could give this a quick review :)

[samselikoff]

Looks good so far! I think for stuff like this

features: () => {},

You could just write

features: {}

But it doesn't really matter

[Turbo87]

@samselikoff the difference is that with () => {} you get a new object for every instance, while with just {} the same object instance is used for every factory object.