issue TryGhost/Ghost#7452, requires TryGhost/Ghost#7451
- use a `ghostOAuth` config flag to switch between the old-style per-install auth and centralized OAuth auth based on config provided by the server
- add OAuth flows for:
- setup
- sign-in
- sign-up
- re-authenticate
- add custom `oauth-ghost` authenticator to support our custom data structure
- add test helpers to stub successful/failed oauth authentication
- hide change password form if using OAuth (temporary - a way to change password via oauth provider will be added later)
closes#7014
- uses the `AjaxServiceSupport` mixin from `ember-ajax` to replace Ember Data's internal `ajax` method with our own ajax service
- normalizes all error handling to use `ember-ajax` style errors
- default to the `application/vnd.api+json` content-type so that we don't have a mix of urlencoded and plain JSON content
- fix `normalizeErrorResponse` in our `ajax` service so that it doesn't add an empty `errors` array to payloads
no issue
- moves mirage config for grouped endpoints into separate files to cleanup the `app/mirage/config.js` file
- endpoints are now mocked via a single `mockX(this)` call allowing for quick/easy mocking in the dev environment as well as the test environment without cluttering `config.js`
