Reworked media API thumbnail endpoint

refs https://github.com/TryGhost/Toolbox/issues/120

- Having an "options" parameter in the controller definition was missleading as if the `url` or `ref` parameters were expected as a part of the qurey parameter. These variables should be provided as a part of the request body, thus having them in "data" attribute is more accurate

core/server/api/canary/media.js

@@ -23,8 +23,9 @@ module.exports = {
 
     uploadThumbnail: {
         permissions: false,
-        options: [
+        data: [
-            'url'
+            'url',
+            'ref'
         ],
         async query(frame) {
             const mediaStorage = storage.getStorage('media');