(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/settings/app_settings.py

@@ -0,0 +1,5 @@
+import settings
+from utils.lib import DotDict
+
+class AppSettings(settings.ObjectSettings):
+    settings = DotDict()