Notify old email when email change is initiated

packages/backend/src/routers/change_email.js

@@ -79,6 +79,11 @@ const CHANGE_EMAIL_START = eggspress('/change_email/start', {
         confirm_url: `${config.origin}/change_email/confirm?token=${jwt_token}`,
         username: user.username,
     });
+    const old_email = user.email;
+    // TODO: NotificationService
+    await svc_email.send_email({ email: old_email }, 'email_change_notification', {
+        new_email: new_email,
+    });
 
     // update user
     await db.write(

packages/backend/src/services/EmailService.js

@@ -63,6 +63,17 @@ We received a request to link this email to the user "{{username}}" on Puter. If
 
 <p>
 <a href="{{confirm_url}}">Confirm email change</a>
+</p>
+                `,
+            },
+            'email_change_notification': {
+                subject: '\u{1f4dd} Notification of email change',
+                html: `
+<p>Hi there,</p>
+<p>
+We're sending an email to let you know about a change to your account.
+We have sent a confirmation to "{{new_email}}" to confirm an email change request.
+If this was not you, please contact support@puter.com immediately.
 </p>
                 `,
             },