Added Feature Flag for ActivityPub

ref MOM-31
ref https://linear.app/tryghost/issue/MOM-31

Ronseal - let's goooooooo

apps/admin-x-settings/src/components/settings/advanced/labs/AlphaFeatures.tsx

@@ -67,6 +67,10 @@ const features = [{
     title: 'Internal Linking',
     description: 'Adds internal URL search to editor link inputs',
     flag: 'internalLinking'
+},{
+    title: 'ActivityPub',
+    description: '(Highly) Experimental support for ActivityPub.',
+    flag: 'ActivityPub'
 }];
 
 const AlphaFeatures: React.FC = () => {

ghost/core/core/shared/labs.js

@@ -38,6 +38,7 @@ const BETA_FEATURES = [
 ];
 
 const ALPHA_FEATURES = [
+    'ActivityPub',
     'NestPlayground',
     'urlCache',
     'lexicalMultiplayer',