Fix unsent activation emails and other asynchronous tasks

Half of the tasks from edx.lms.core.default celery queue were being
processed by the CMS worker. Unfortunately, this CMS worker crashes on
some of those tasks. For instance, activation emails complain of a
missing "django_markup" template tag library because "xss_utils" is not
part of the installed app in the CMS.

The problem is that we need this edx.lms.core.default queue to be part
of the CELERY_QUEUES in the cms in order to send tasks from the CMS to
the LMS. The trick to resolve this situation is to ask the CMS celery
worker to not process the tasks from this queue.

To debug this issue, run in the LMS:

    from student.tasks import send_activation_email
    send_activation_email("{}")

Then watch the logs of the lms and cms workers. If the CMS workers picks
up this task (50% of the time prior to this change) then we have an
issue.

See:
https://discuss.overhang.io/t/reset-password-email-sent-but-activation-email-dont/690

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 Note: Breaking changes between versions are indicated by "💥".
 
+## Unreleased
+
+- [Bugfix] Fix unsent activation emails and other asynchronous tasks
+
 ## v10.0.5 (2020-06-21)
 
 - [Security] Apply edx-platform upstream xss security fixes ([pull request](https://github.com/edx/edx-platform/pull/24258))

tutor/templates/local/docker-compose.yml

@@ -151,7 +151,7 @@ services:
       SERVICE_VARIANT: cms
       SETTINGS: ${EDX_PLATFORM_SETTINGS:-tutor.production}
       C_FORCE_ROOT: "1" # run celery tasks as root #nofear
-    command: ./manage.py cms celery worker --loglevel=info --hostname=edx.cms.core.default.%%h --maxtasksperchild 100
+    command: ./manage.py cms celery worker --loglevel=info --hostname=edx.cms.core.default.%%h --maxtasksperchild 100 --exclude-queues=edx.lms.core.default
     restart: unless-stopped
     volumes:
       - ../apps/openedx/settings/lms/:/openedx/edx-platform/lms/envs/tutor/