Revert "Use named URLs in get_email_link"

This reverts commit 849ff973adc232d0bf55ac44350b7ed72605bd59.

This change, unfortunately, was broken.

Typical Karaage install, we have two sites:

https://www.example.org/kgadmin/...
https://www.example.org/users/...

If an administrator logs into the admin site (kgadmin), and creates an
application for a user, it will create an email to send to the user.  As
a result, it must contain a link that is usable by the user, not the
administrator (users).

However, Karaage, after this change, when run from the admin site, will
create the URL as follows:

url = reverse('kg_application_unauthenticated', args=[application.secret_token])
    = "/kgadmin/applications/%s" % application.secret_token

url = '%s%s' % (settings.REGISTRATION_BASE_URL, url)
    = "https://www.example.org/users/kgadmin/applications/%s"

Which unfortunately, is wrong, and will generate a HTTP 404.

Unfortunately, we cannot use reverse to construct URLS, for what is
another website (even if it is another instance of Karaage).

Also, even if the email is generated from the users site, the URL still ends
up being wrong:

url = reverse('kg_application_unauthenticated', args=[application.secret_token])
    = "/users/applications/%s" % application.secret_token

url = '%s%s' % (settings.REGISTRATION_BASE_URL, url)
    = "https://www.example.org/users/users/applications/%s"

This will also generate a HTTP 404.

Change-Id: Ica9e4aa3a283240e80b12726c2a0ac9cbc7efca3