Add more instance stats APIs (#6125)

* Add GET /api/v1/instance/peers API to reveal known domains

* Add GET /api/v1/instance/activity API

* Make new APIs disableable, exclude private statuses from activity stats

* Fix code style issue

* Fix week timestamps

app/controllers/admin/settings_controller.rb

@@ -17,6 +17,8 @@ module Admin
       bootstrap_timeline_accounts
       thumbnail
       min_invite_role
+      activity_api_enabled
+      peers_api_enabled
     ).freeze
 
     BOOLEAN_SETTINGS = %w(
@@ -24,6 +26,8 @@ module Admin
       open_deletion
       timeline_preview
       show_staff_badge
+      activity_api_enabled
+      peers_api_enabled
     ).freeze
 
     UPLOAD_SETTINGS = %w(