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/settings/_schema.py

@@ -0,0 +1,12 @@
+from libretime_shared.config import (
+    BaseConfig,
+    DatabaseConfig,
+    GeneralConfig,
+    RabbitMQConfig,
+)
+
+
+class Config(BaseConfig):
+    general: GeneralConfig
+    database: DatabaseConfig = DatabaseConfig()
+    rabbitmq: RabbitMQConfig = RabbitMQConfig()