Reverted destroy function of CRUD plugin to chained promises

refs https://github.com/TryGhost/Team/issues/808

- see referenced issue for context, but turning this function into
  async-await seems to have broken error handling when deleting things
  that don't exist
- i'm really not sure why - but my running theory is that it's something
  to do with Bluebird Promises vs native Promises
- this should keep the same functionality until I can investigate what
  is going on

core/server/models/base/plugins/crud.js

@@ -198,7 +198,7 @@ module.exports = function (Bookshelf) {
          * @param {Object} [unfilteredOptions]
          * @return {Promise<Bookshelf['Model']>} Empty Model
          */
-        destroy: async function destroy(unfilteredOptions) {
+        destroy: function destroy(unfilteredOptions) {
             const options = this.filterOptions(unfilteredOptions, 'destroy');
 
             if (!options.destroyBy) {
@@ -208,8 +208,11 @@ module.exports = function (Bookshelf) {
             }
 
             // Fetch the object before destroying it, so that the changed data is available to events
-            const obj = await this.forge(options.destroyBy).fetch(options);
-            return obj.destroy(options);
+            return this.forge(options.destroyBy)
+                .fetch(options)
+                .then(function then(obj) {
+                    return obj.destroy(options);
+                });
         },
 
         // When loading an instance, subclasses can specify default to fetch