Upgrade gunicorn to 19.6.0

Includes a note about why gevent has been left at 1.0.2. ECOM-5386.

Update basket deletion management command

Avoid deletion of invoiced baskets, and sleep between each batch deletion. Also do a better job explaining why we used batched deletes. In service of ECOM-5386.

Revert "Upgrade gunicorn and gevent"

This reverts commit 20eec8e0. I'm backing this out because of a problem with the way gevent wraps Python's __import__ functionality.

Really fix missing JSON data.

Upgrade gunicorn and gevent

Byproduct of ECOM-5386.

Check existence of JSON data from both catalog and LMS course API

Revert use of catalog for basket page.

This reverts commit b1250ba5.

This reverts commit 72386e07.

Check for top level image null

Use catalog service for course info.

Fixed course save button not redirecting

[SOL-2004] Display seats instead of courses in catalog preview API endpoint.

Rename start and end datetime parameters; Refactor update dictionary creation method;

[SOL-2038] Add a check if email_domains are in coupon update data before updating it

Remove API constants

Fix Coupon Acceptance Tests

Add a list of email domains to an offer for validation at redemption.

Updated translations

[SOL-2033] Change voucher list to raw ID list in basket details admin page.

SOL-1991: Create Management Command for adding Site and SiteThemes