Make separate production and staging versions of cron.txt

[jacobdgm]

This PR:

• renames cron-staging.txt -> cron.txt.staging, and renames cron.txt -> cron.txt.production, thus bringing our cron configuration into alignment with our docker (in docker-compose.yml) and nginx (in conf.d/cantusdb.conf) configuration.
• Adds cron/cron.txt to .gitignore

This PR fully resolves #1269.

[dchiller]

Looks good. We can reduce a deployment step...these don't need to be cat-ed anywhere before they are added to crontab, but ok as is.