Fix django-cron jobs

[suecarmol]

Description

Added the retry_after_failure_mins in the schedule constructor.

Rationale

The crons will retry more frequently after failures, which can cause a server overload.

Phabricator Ticket

T350295

How Has This Been Tested?

Screenshots of your changes (if appropriate):

Types of changes

What types of changes does your code introduce? Add an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

[jsnshrmn]

Okay, I actually tested cron failures this time. I did the following:

• set backup script to exit with status 1
• ran the backup cron every two seconds
• watch docker-compose exec externallinks python manage.py runcrons 'extlinks.common.cron.BackupCron'
• watched the log fill up with failed backup jobs on master
• watched the job get skipped until the failutre interval (which I shortened for this test) in your patch.