[APER-1941]
We are aware of a product issue that causes a `certificate_available_date` (CAD) to be set for self-paced courses (and thus copied to the course-run's (course) certificate configuration) that causes an issue with learners' Program Records to be inaccurate. The stored CAD in Credentials is causing these certificates to be marked as "unearned" on the Program Record in Credentials, as the IDA believes the learner should *not* have access to them yet (but these certificates are available in the LMS).
A management command was recently introduced in Studio that can be used to clean/remove the `certificate_available_date` data from a course-run in Mongo. These updates aren't making it to the Credentials IDA because of an issue with our logic in the `update_certificate_visible_date_on_course_update` Celery task. This task assumes that we only want to send updates for *Instructor-Paced* courses that have a Certificate Display Behavior set to `end_with_date`. In reality, we need updates to pass to Credentials for _some_ self-paced courses with bad data.
This PR hopes to update our infrastructure to allow these updates to flow to Credentials.
* Improve logging for failed requests to the Credentials IDA's `course_certificates` endpoint when updating a course certificate configuration.
* Update docstrings and comments where appropriate
* Split the logic of the update_certificate_visible_date_on_course_update task into two tasks. The former task will continue to _just_ handle visible_date attribute updates. The latter (new) task will be dedicated to making the REST API call that updates the `certificate_available_date` data in Credentials.
* Update the `handle_course_cert_date_change` function wqhen the COURSE_CERT_DATE_CHANGE signal is received to queue both the "visible_date" and "certificate available date" Celery tasks.
* Update existing tests for the task changes.
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