Generate truly random email addresses in factory

Previously, it could happen under some (random) circumstances that a series of users got the same user name. This is caused by the usage of Forgery for generating a name. Forgery itself picks a random name from a list, and there is no check that two calls would indeed deliver two different names.

In order to solve the issue, we generate a truly unique email address for specs, so that the problem won't arise again.

spec/factories/shared_traits.rb

@@ -8,7 +8,7 @@ FactoryBot.define do
   end
 
   trait :generated_email do
-    email { "#{name.underscore.tr(' ', '.')}@example.org" }
+    sequence(:email) {|n| "#{name.underscore.tr(' ', '.')}.#{n}@example.org" }
   end
 
   trait :generated_user_name do