subdomains: Use subdomain host in password reset emails.

templates/registration/password_reset_email.txt

@@ -1,8 +1,8 @@
-Psst. Word on the street is that you forgot your password, {{ email}}.
+Psst. Word on the street is that you forgot your password, {{ email }}.
 
 It's all good. Follow the link below and we'll take care of the rest:
 
-{{ protocol}}://{{ domain }}{{ url('django.contrib.auth.views.password_reset_confirm', kwargs=dict(uidb64=uid, token=token)) }}
+{{ protocol }}://{{ user.realm.host }}{{ url('django.contrib.auth.views.password_reset_confirm', kwargs=dict(uidb64=uid, token=token)) }}
 
 Thanks,
 Your friends at Zulip HQ

zerver/tests/test_templates.py

@@ -88,6 +88,7 @@ class TemplateTestCase(ZulipTestCase):
         context = dict(
             shallow_tested=True,
             user_profile=user_profile,
+            user=user_profile,
             product_name='testing',
             form=DummyForm(
                 full_name=get_form_value('John Doe'),