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/permissions.py

@@ -55,7 +55,7 @@ def check_authorization_header(request):
 
     if auth_header.startswith("Api-Key"):
         token = auth_header.split()[1]
-        if token == settings.CONFIG.get("general", "api_key"):
+        if token == settings.CONFIG.general.api_key:
             return True
     return False