refs https://github.com/TryGhost/Admin/pull/2209
- `miragejs` has been extracted to a framework-independent library, the re-exports of `miragejs` elements in `ember-cli-mirage` have been deprecated making our test logs very noisy
- added `miragejs` as a top-level dependency
- updated all relevant imports to pull from `miragejs` instead of `ember-cli-mirage`
no issue
- moved navigation settings tests from old settings/design tests to a separate file and unskipped them
- added happy-path acceptance tests for
- rendering design screen
- installing an official theme from the themes list
- uploading a custom theme