Add created_at to schools dashboard index, default order by this

Author: grega

app/dashboards/school_dashboard.rb

@@ -38,6 +38,7 @@ class SchoolDashboard < Administrate::BaseDashboard
     name
     reference
     country_code
+    created_at
     verified_at
     rejected_at
   ].freeze
@@ -95,4 +96,12 @@ class SchoolDashboard < Administrate::BaseDashboard
   def display_resource(school)
     school.name.to_s
   end
+
+  def default_sorting_attribute
+    :created_at
+  end
+
+  def default_sorting_direction
+    :desc
+  end
 end