Add delay to course email task and use SITE_NAME for site url

 Delay for possible race condition with fetching course email object.
 Use settings.SITE_NAME for host name to generate email footer url.

lms/djangoapps/bulk_email/tasks.py

@@ -63,7 +63,7 @@ def delegate_email_batches(hash_for_msg, recipient, course_id, course_url, user_
 
     for i in range(num_workers):
         to_list = recipient_list[i * chunk:i * chunk + chunk]
-        course_email.delay(hash_for_msg, to_list, course.display_name, course_url, False)
+        course_email.apply_async(args=[hash_for_msg, to_list, course.display_name, course_url, False], countdown=10)
     return num_workers
 
 

lms/templates/emails/email_footer.html

@@ -3,4 +3,4 @@
 ----<br />
 This email was automatically sent from ${settings.PLATFORM_NAME} to ${name}. <br />
 You are receiving this email at address ${ email } because you are enrolled in <a href="${course_url}">${ course_title }</a>.<br />
-To stop receiving email like this, update your course email settings <a href="https://${site}${reverse('dashboard')}">here</a>. <br />
+To stop receiving email like this, update your course email settings <a href="https://${settings.SITE_NAME}${reverse('dashboard')}">here</a>. <br />

lms/templates/emails/email_footer.txt

@@ -4,4 +4,4 @@
 This email was automatically sent from ${settings.PLATFORM_NAME} to ${name}.
 You are receiving this email at address ${ email } because you are enrolled in ${ course_title }
 (URL: ${course_url} ).
-To stop receiving email like this, update your account settings at https://${site}${reverse('dashboard')}.
+To stop receiving email like this, update your account settings at https://${settings.SITE_NAME}${reverse('dashboard')}.