Added serializer for offers

refs: https://github.com/TryGhost/Toolbox/issues/245

- Offers didn't have a serializer because it's deliberately set at the API level
- With the upcoming refactor we want to have all the serializers defined explicitly
- This will allow us to change the default behaviour
- Tests were added to cover this endpoint in ...

core/server/api/canary/utils/serializers/output/index.js

@@ -163,5 +163,9 @@ module.exports = {
 
     get session() {
         return require('./session');
+    },
+
+    get offers() {
+        return require('./offers');
     }
 };

core/server/api/canary/utils/serializers/output/offers.js

@@ -0,0 +1,8 @@
+const debug = require('@tryghost/debug')('api:canary:utils:serializers:output:offers');
+
+module.exports = {
+    all() {
+        debug('all');
+        // Offers has frame.response already set
+    }
+};