Added hacky method for accessing meta value of single-record requests

no issue

- Ember Data does not support accessing meta data included in the response to single-record requests such as save/delete
- approach to allow it taken from emberjs/data#4077 (comment)

app/models/base.js

@@ -0,0 +1,12 @@
+import Model from 'ember-data/model';
+
+export default Model.extend({
+    // this is a hack that gives us access to meta data in single resource responses
+    // allows similar response.get('_meta') as with multi-resource responses but can
+    // suffer from race conditions
+    // TODO: review once the record links and meta RFC lands
+    // https://github.com/emberjs/rfcs/blob/master/text/0332-ember-data-record-links-and-meta.md
+    get _meta() {
+        return this._internalModel.type.___meta;
+    }
+});

app/models/user.js

@@ -1,5 +1,5 @@
 /* eslint-disable camelcase */
-import Model from 'ember-data/model';
+import BaseModel from './base';
 import ValidationEngine from 'ghost-admin/mixins/validation-engine';
 import attr from 'ember-data/attr';
 import {computed} from '@ember/object';
@@ -8,7 +8,7 @@ import {hasMany} from 'ember-data/relationships';
 import {inject as service} from '@ember/service';
 import {task} from 'ember-concurrency';
 
-export default Model.extend(ValidationEngine, {
+export default BaseModel.extend(ValidationEngine, {
     validationType: 'user',
 
     name: attr('string'),

app/serializers/application.js

@@ -3,6 +3,16 @@ import {camelize, decamelize, underscore} from '@ember/string';
 import {pluralize} from 'ember-inflector';
 
 export default RESTSerializer.extend({
+    // hacky method for getting access to meta data for single-resource responses
+    // https://github.com/emberjs/data/pull/4077#issuecomment-200780097
+    // TODO: review once the record links and meta RFC lands
+    // https://github.com/emberjs/rfcs/blob/master/text/0332-ember-data-record-links-and-meta.md
+    extractMeta(store, typeClass) {
+        let meta = this._super(...arguments);
+        typeClass.___meta = meta;
+        return meta;
+    },
+
     serialize(/*snapshot, options*/) {
         let json = this._super(...arguments);