REFACTOR: Move option to return emails into the serializer

This makes more sense than having the guardian take an accessor.
The logic belongs in the Serializer, where the JSON is calculated.

Also removed some of the DRYness in the spec. It's fewer lines
and made it easier to test the option on the serializer.

app/controllers/admin/users_controller.rb

@@ -32,12 +32,13 @@ class Admin::UsersController < Admin::AdminController
   def index
     users = ::AdminUserIndexQuery.new(params).find_users
 
+    opts = {}
     if params[:show_emails] == "true"
-      guardian.can_see_emails = true
       StaffActionLogger.new(current_user).log_show_emails(users)
+      opts[:emails_desired] = true
     end
 
-    render_serialized(users, AdminUserListSerializer)
+    render_serialized(users, AdminUserListSerializer, opts)
   end
 
   def show