[IMP] website_slides_survey: handle certification re-enroll/purchase flow

Task #1945036

Purpose
=======

If the user fails his last attempt at a course certification, we remove him from
the members of the course (and he has to enroll again).
He receives an email in the process notifying him of his failure and suggesting
he enrolls to the course again.

The purpose is to have a 'certification flow' where the user can re-purchase the
certification when they have failed it.

This could lead to some issues if the course containing the certification also has
other slides with content because the user will not have access to them after failing.
This also prevents configuring courses with multiple certifications since the membership
will be removed at the first failure.

These use cases are considered "non standard" by the business and are thus not handled
in the code. We assume that users will configure their courses "correctly".