feat: switching to celery native backoff for cert awarding (openedx#3…

…5009)

* 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