(core): Add app-global setting storage.

New `AppConfig` key-value table for arbitrary app config.
New `KeyValueData` setting data mixin.
New `AppSettings` settings group.
Attached `AppSettings` as `client.settings`.
Migrated sponsor settings to `AppSettings`.

bot/main.py

@@ -4,6 +4,9 @@ from data import tables
 
 import core # noqa
 
+# Note: This MUST be imported after core, due to table definition orders
+from settings import AppSettings
+
 import modules  # noqa
 
 # Load and attach app specific data
@@ -15,6 +18,8 @@ client.appdata = core.data.meta.fetch_or_create(appname)
 
 client.data = tables
 
+client.settings = AppSettings(conf.bot['data_appid'])
+
 # Initialise all modules
 client.initialise_modules()