Fixes #1622 - split the api into 4 apps: core, history, schedule, storage - exploded the settings into testing/prod
- add django settings module documentation - use default for previously required fields BREAKING CHANGE: The API command line interface require the configuration file to be present. The default configuration file path is `/etc/airtime/airtime.conf`
- add --py3-plus flag to pyupgrade hook - add --py36-plus flag to pyupgrade hook