feat(api): load config using shared helpers

- 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`

api/libretime_api/cli.py

@@ -7,6 +7,7 @@ import sys
 
 def main():
     os.environ.setdefault("DJANGO_SETTINGS_MODULE", "libretime_api.settings")
+    os.environ.setdefault("LIBRETIME_CONFIG_FILEPATH", "/etc/airtime/airtime.conf")
     try:
         from django.core.management import execute_from_command_line
     except ImportError as exc: