Added serializer for slack.all

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

- Slack didn't have a serializer because it's deliberately a passthrough
- With the upcoming refactor we want to have all the serializers defined explicitly
- This will allow us to change the default behaviour
- Tests already cover this endpoint

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

@@ -155,5 +155,9 @@ module.exports = {
 
     get custom_theme_settings() {
         return require('./custom-theme-settings');
+    },
+
+    get slack() {
+        return require('./slack');
     }
 };

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

@@ -0,0 +1,9 @@
+const debug = require('@tryghost/debug')('api:canary:utils:serializers:output:slack');
+
+module.exports = {
+    all(themes, apiConfig, frame) {
+        debug('all');
+
+        frame.response = themes;
+    }
+};