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/models/form/admin_settings.rb

@@ -30,6 +30,10 @@ class Form::AdminSettings
     :bootstrap_timeline_accounts=,
     :min_invite_role,
     :min_invite_role=,
+    :activity_api_enabled,
+    :activity_api_enabled=,
+    :peers_api_enabled,
+    :peers_api_enabled=,
     to: Setting
   )
 end