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

config/settings.yml

@@ -47,7 +47,8 @@ defaults: &defaults
     - webmaster
     - administrator
   bootstrap_timeline_accounts: ''
-
+  activity_api_enabled: true
+  peers_api_enabled: true
 development:
   <<: *defaults