 a
link the user can click to activate the account. This template has the
following context:

``activation_key``
    The activation key for the new account.

``expiration_days``
    The number of days remaining during which the account may be
    activated.

``site``
    An object representing the site on which the user registered;
    depending on whether ``django.contrib.sites`` is installed, this
    may be an instance of either ``django.contrib.sites.models.Site``
    (if the sites application is instal