Replace formatted translatable strings with templates for better flexibility

[LilyFoote]

The password_reset_email_handler and validation_email_handler each define a subject that is formatted with the site.domain. This is difficult to override when the domain is unwanted.

We could instead use a django template, to which we pass the site.domain in a context. This would allow a project to replace the subject completely, ignoring the context.

[kevinetienne]

Can the subject be passed through the notification? Or be overridden by the notification if present?