Using context variable as html attribute in django template

I've cloned parts of django-registration into my project. Specifically, the RegistrationProfile bit for activation_key generation, user activation, and verification email sending. I've replaced the native send_email bit with a EmailMultiAlternatives version to send HTML Emails. It look something like this:

def send_activation_email(self, site):
    context = {
        'link': (site+"signup/activate/"+self.activation_key),
        'expiration_days': settings.ACCOUNT_ACTIVATION_DAYS,
        'site': site
    }
    # SUBJECT
    subject = render_to_string(
        'registration/activation_email_subject.txt',
        context
    )
    subject = ''.join(subject.splitlines())
    # BODY
    html_content = render_to_string(
        'registration/activation_email.html',
        context
    )
    text_content = strip_tags(html_content)
    # MESSAGE
    message = EmailMultiAlternatives(
        subject,
        text_content,
        settings.DEFAULT_FROM_EMAIL,
        [self.user.email]
    )
    message.attach_alternative(html_content, "text/html")

I can print the link with {{link}} in the template. How can I use the link as an href attribute if I wanted something like <a href={{link}}>Activate</a> (or a button)?