Catch exceptions when deleting remove public keys

References [Task] https://code.ornl.gov/sns-hfir-scse/reflectometry/reflectometry/-/issues/421

Work Done

• auxiliary functions to deal with exceptions when deleted public keys from authorized_keys file in the remote fit worker
• using the auxiliary functions when logging out and when purging expired sessions
• custom manage.py command to purge expired functions (python manage.py sessions --purge --expired-only)

Manual Testing

in the LOCAL environment:

• login; logout.
• login; run manual purge of all sessions with with the manage.py sessions command
• set a 1 minute for session lifetime, then:
  • login
  • wait for the recursive celery task that expunges expired sessions to run, hence deleting the session
  • enter the admin website and delete the recursive celery task
  • login
  • wait and run manual purge of expired sessions with the manage.py sessions command