refs #6039
- adds ember-cli-mirage dependency
- sets up mirage to match our API endpoints and responses
- adds fixture data for settings that are always present for all blogs
- converts existing acceptance tests to use mirage
closes#5852
- resets navigation settings controller's model when transitioning away
- fixes `locationType` config setting so acceptance tests don't mess with the URL
- configure the ephemeral session store for ember-simple-auth during tests
- adds dummy env-config meta fields so acceptance tests don't fail
- adds `ember-cli-simple-auth-testing` dependency for auth testing helpers
- adds Pretender dependency to mock API requests for acceptance tests
