emails: Fix notify_new_login emails have unserializable context.

Regression introduced in e0f30d0 (PR #7569).

templates/zerver/emails/notify_new_login.source.html

@@ -10,7 +10,7 @@
 <p><b>Login details:</b></p>
 <blockquote>
     <p>Server: <a href="{{ realm_uri }}" target="_blank">{{ realm_uri }}</a></p>
-    <p>Account: <a href="mailto:{{ user.email }}" target="_blank">{{ user.email }}</a></p>
+    <p>Account: <a href="mailto:{{ user_email }}" target="_blank">{{ user_email }}</a></p>
     <p>Time: {{ device_info.login_time }}</p>
     <p>Device: {{ device_info.device_browser if device_info.device_browser else 'An unknown browser' }} on {{ device_info.device_os if device_info.device_os else 'an unknown operating system' }}.</p>
     <p>IP Address: {{ device_info.device_ip }}</p>

templates/zerver/emails/notify_new_login.txt

@@ -4,7 +4,7 @@ This is a notification that a new login to your Zulip account has just occurred.
 
 Login details:
 Server: {{ realm_uri }}
-Account: {{ user.email }}
+Account: {{ user_email }}
 Time: {{ device_info.login_time }}
 Device: {{ device_info.device_browser if device_info.device_browser else 'an unknown browser' }} on {{ device_info.device_os if device_info.device_os else 'an unknown operating system' }}.
 IP Address: {{ device_info.device_ip }}

zerver/signals.py

@@ -81,7 +81,7 @@ def email_on_new_login(sender: Any, user: UserProfile, request: Any, **kwargs: A
 
         context = common_context(user)
         context['device_info'] = device_info
-        context['user'] = user
+        context['user_email'] = user.email
 
         email_dict = {
             'template_prefix': 'zerver/emails/notify_new_login',