Add the ability to send emails to devs of paid apps (bug 896034)

… that checked the `enable_new_regions` checkbox or not.

Known limitation: you can't pass variables from the admin interface and thus the `region name` or `app name` can't be dynamically populated. On the other side, with the newly added `email_developers_about_new_paid_region` command it's maybe less relevant(?).

To remove that limitation, we need to create a new command that can't be handled via zamboni's admin.