(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/core/data.py

@@ -11,6 +11,9 @@ meta = RowTable(
     attach_as='meta',
 )
 
+# TODO: Consider converting to RowTable for per-shard config caching
+app_config = Table('AppConfig')
+
 
 user_config = RowTable(
     'user_config',