Adds `EDXAPP_ADDITIONAL_CRON_JOBS`, which provides a fully configurable list of cron jobs to be added.

Default is an empty list, so no additional cron jobs are added.

*JIRA tickets*: SE-126, https://openedx.atlassian.net/browse/OSPR-3007#icft=OSPR-3007(https://openedx.atlassian.net/browse/OSPR-3007)

*Discussions*: See https://github.com/edx/edx-platform/pull/19047

*Sandbox URL*: Sandbox being provisioned.

• LMS: https://pr19047.sandbox.opencraft.hosting/

• Studio: https://studio-pr19047.sandbox.opencraft.hosting/

Running [edx-platform/master](https://github.com/edx/edx-platform/commit/33fab4b1e4a5f7d3b388bc8105c6b60abd1d93e0), configured using this branch and the extra configuration below. Has 2 active VMs (`vm1.pr19047.sandbox.opencraft.hosting`, and `vm2.pr19047.sandbox.opencraft.hosting`)

*Merge deadline*: Would like to get this merged before Ironwood if possible.

*Testing instructions*:

1. On the sandbox, register and activate an account.
1. Enrol in the [Weekly Course Updates test course](https://pr19047.sandbox.opencraft.hosting/courses/course-v1:OpenCraft+Highlights+2018_1/about)
1. With the current configuration, you'll get a scheduled course update in your inbox in a week.

To trigger the email earlier:
1. Login to the sandbox as the `staff` user (it's been elevated to superuser).
1. Visit the [Django Admin Schedules › Schedules ](https://pr19047.sandbox.opencraft.hosting/admin/schedules/schedule) and locate the schedule created for your newly-registered user above.
Adjust the Start date to backdate it by 1 week. You should get a single email for Week 1 in your inbox within 24 hours.

*Reviewers*

• [x] @clemente

• [ ] edX DevOps reviewer[s] TBD

*Settings*
```yaml
EDXAPP_ADDITIONAL_CRON_JOBS:

• name: "Send weekly course highlight emails"
  user: "{{ edxapp_user }}"
  job: "VM1=$(dig vm1.{{ COMMON_DEPLOYMENT }} +short -4); /sbin/ifconfig | grep \"inet addr:${VM1} \" && . {{ edxapp_app_dir }}/edxapp_env && {{ edxapp_venv_bin }}/python {{ edxapp_code_dir }}/manage.py lms send_course_update --settings={{ edxapp_settings }} {{ EDXAPP_SITE_NAME }} >/dev/null 2>&1"
  hour: "0"
  minute: "0"
  day: "*"

configuration_repo_url: https://github.com/open-craft/configuration
configuration_version: jill/addl-crons

1.  

   1. Use default django mailer for scheduled course highlight messages
      EDXAPP_ACE_CHANNEL_DEFAULT_EMAIL: "django_email"
      EDXAPP_ACE_CHANNEL_SAILTHRU_DEBUG: false
      EDXAPP_ACE_CHANNEL_SAILTHRU_TEMPLATE_NAME: "template"
      EDXAPP_ACE_CHANNEL_TRANSACTIONAL_EMAIL: "django_email"
      EDXAPP_ACE_ENABLED_CHANNELS: ["django_email"]
      EDXAPP_ACE_ENABLED_POLICIES: ["bulk_email_optout"]
      EDXAPP_DEFAULT_FROM_EMAIL: “jill+highlights@opencraft.com”
      ```

Configuration Pull Request
—

Make sure that the following steps are done before merging:

• [ ] A DevOps team member has approved the PR if it is code shared across multiple services and you don't own all of the services.

• [x] Are you adding any new default values that need to be overridden when this change goes live? No

• [ ] <strike>Update the appropriate internal repo (be sure to update for all our environments)</strike>

• [ ] <strike>If you are updating a secure value rather than an internal one, file a DEVOPS ticket with details.</strike>

• [ ] <strike>Add an entry to the CHANGELOG.</strike>

• [x] Are making a complicated change? No