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/api/activitypub/activities_controller.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+class Api::Activitypub::ActivitiesController < ApiController
+  # before_action :set_follow, only: [:show_follow]
+  before_action :set_status, only: [:show_status]
+
+  respond_to :activitystreams2
+
+  # Show a status in AS2 format, as either an Announce (reblog) or a Create (post) activity.
+  def show_status
+    headers['Access-Control-Allow-Origin'] = '*'
+
+    return forbidden unless @status.permitted?
+
+    if @status.reblog?
+      render :show_status_announce
+    else
+      render :show_status_create
+    end
+  end
+
+  private
+
+  def set_status
+    @status = Status.find(params[:id])
+  end
+end