ActivityPub: Add basic, read-only support for Outboxes, Notes, and Create/Announce Activities (#2197)

* Clean up collapsible components

* Expose user Outboxes and AS2 representations of statuses

* Save work thus far.

* Fix bad merge.

* Save my work

* Clean up pagination.

* First test working.

* Add tests.

* Add Forbidden error template.

* Revert yarn.lock changes.

* Fix code style deviations and use localized instead of hardcoded English text.

app/controllers/accounts_controller.rb

@@ -15,7 +15,9 @@ class AccountsController < ApplicationController
         render xml: AtomSerializer.render(AtomSerializer.new.feed(@account, @entries.to_a))
       end
 
-      format.activitystreams2
+      format.activitystreams2 do
+        headers['Access-Control-Allow-Origin'] = '*'
+      end
     end
   end