Add alternate links to ActivityPub resources from HTML/HEAD variants (#4586)

app/controllers/concerns/account_controller_concern.rb

@@ -23,6 +23,7 @@ module AccountControllerConcern
       [
         webfinger_account_link,
         atom_account_url_link,
+        actor_url_link,
       ]
     )
   end
@@ -41,6 +42,13 @@ module AccountControllerConcern
     ]
   end
 
+  def actor_url_link
+    [
+      ActivityPub::TagManager.instance.uri_for(@account),
+      [%w(rel alternate), %w(type application/activity+json)],
+    ]
+  end
+
   def webfinger_account_url
     webfinger_url(resource: @account.to_webfinger_s)
   end