Clean up admin UI for accounts (#6004)

* Add staff filter to admin UI for accounts, remove obsolete columns

* Only display OStatus section in admin UI for accounts when OStatus data

app/controllers/admin/accounts_controller.rb

@@ -89,7 +89,8 @@ module Admin
         :username,
         :display_name,
         :email,
-        :ip
+        :ip,
+        :staff
       )
     end
   end