* feat: switching to celery native backoff for cert awarding
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
* feat: switching to celery native backoff for cert awarding
* Fixed the grammar in a couple of comments
* fixed a couple of lending errors
FIXES: APER-3510
* feat: limiting PII in logs per code review
per code review
FIXES: APER-3510
* feat: code review feedback
* removing some more PII from logs, even where it was not requested
* circuit breaker returned from grading attempt if course key is bad
* add missing tests for that functionality
FIXES: APER-3510
* feat: improved logging
per code review, adding a log message explaining certificate mode
issues
FIXES: APER-3510
* feat: fixing a bad string
lint error, and left off the format string modifier
FIXES: APER-3510
* feat: style
linter error
FIXES: APER-3510
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