feat: config setting to automatically set members/groups to private when creating

PluralKit.Core/Database/Migrations/22.sql

@@ -0,0 +1,7 @@
+-- schema version 22
+-- automatically set members/groups as private when creating
+
+alter table system_config add column member_default_private bool not null default false;
+alter table system_config add column group_default_private bool not null default false;
+
+update info set schema_version = 22;