'Activate an :class:`User` by supplying a valid ``activation_key``.
If the key is valid and an user is found, activates the user and
return it. Also sends the ``activation_complete`` signal.
:param activation_key:
String containing the secret SHA1 for a valid activation.
:return:
The newly activated :class:`User` or ``False`` if not successful.'

def activate_user(self, activation_key):

    if SHA1_RE.search(activation_key):
        try:
            userena = self.get(activation_key=activation_key)
        except self.model.DoesNotExist:
            return False
        if (not userena.activation_key_expired()):
            userena.activation_key = userena_settings.USERENA_ACTIVATED
            user = userena.user
            user.is_active = True
            userena.save(using=self._db)
            user.save(using=self._db)
            userena_signals.activation_complete.send(sender=None, user=user)
            return user
    return False
