Add new CronJobRun model; add instances when cron jobs are executed

[pep8speaks]

Hello @acrellin! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file app/models.py:

Line 146:80: E501 line too long (82 > 79 characters) Line 180:80: E501 line too long (81 > 79 characters)

• In the file services/cron/cron.py:

Line 89:80: E501 line too long (88 > 79 characters) Line 92:80: E501 line too long (113 > 79 characters)

Comment last updated at 2021-02-22 23:59:51 UTC

[acrellin]

@stefanv this is the PR I was referring to -- whenever you have a moment, I'd love a review

[acrellin]

Thanks, @stefanv !