Close government / update ministers transactionally

Currently, it's possible for this method to close the government, and
then run into trouble later on while closing ministerial appointments.
If one of these `save!` calls failed, we'd be left in an inconsistent
state.

Wrapping the update and save! calls in a transaction will mean that they
either all succeed, or are all rolled back, avoiding the chance of
getting into this bad state.

I also suspect (but am not 100% sure) that doing this transactionally
will significantly speed up the code, because we won't have to open /
close a transaction for every role appointment we save.

app/controllers/admin/governments_controller.rb

@@ -42,11 +42,13 @@ def prepare_to_close
   def close
     government = Government.find(params[:id])
 
-    government.update!(end_date: Time.zone.today) unless government.end_date
+    ActiveRecord::Base.transaction do
+      government.update!(end_date: Time.zone.today) unless government.end_date
 
-    current_active_ministerial_appointments.each do |appointment|
-      appointment.ended_at = government.end_date
-      appointment.save!(validate: false)
+      current_active_ministerial_appointments.each do |appointment|
+        appointment.ended_at = government.end_date
+        appointment.save!(validate: false)
+      end
     end
 
     redirect_to edit_admin_government_path(government), notice: "Government closed"