We were assigning variants in a controller, in the context of a request.
However, when sending emails, there is no request and no controller is
involved, so we also need to set the variant in the ApplicationMailer
class.
We were duplicating the asset host and the URL host in all environments,
but we can make it so the asset host uses the URL host unless we
specifically set it.
Note that, inside the `ApplicationMailer`, the `root_url` method already
uses `default_url_options` to generate the URL.
In the rare case of CONSUL installations who have changed the asset
host, this change has no effect since they'll get a conflict in the
environment files when upgrading and they'll choose to use their own
asset host.
