feat: switching to celery native backoff for cert awarding

[deborahgu]

Description

Our homegrown backoff/retry was good enough for a while, but we ran into a huge disabling event when too many changes were made simultaneously. Since this code was first written, celery has built in good back off/retry functionality, including jitter, to make sure that all the retries don't happen simultaneously.

• Switches to using celery native backoff
• Refactors a huge try/catch block so the exception handling is on smaller subsets of code

FIXES: APER-3510

Testing instructions

Using a course with many enrollees, change a course overview in a way that will trigger a new run of award/revoke course certificates.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.