Cron interval

[jamesros161]

Resolves #584

Test File

boldgrid-backup.1.16.0-rc.1.zip

Testing Instructions

1. Install the test zip file.
2. In the Total Upkeep settings page, set the cron interval
3. Setup a remote storage such as Google Drive
4. Setup an automatic backup to take place.
5. Check the crontab in your terminal to verify that the interval for run-jobs.php matches what you selected.
6. Once the Automatic backup finishes, a job should get added to the jobs list to upload the file to google drive.
7. Ensure that the cron runs at the appropriate interval.
8. Once the jobs list is completed, on the following interval, the cron job should be removed from the crontab. Check it to ensure this is the case.