Add per-system member/group limit override

PluralKit.Core/Database/Migrations/10.sql

@@ -0,0 +1,11 @@
+-- SCHEMA VERSION 10: 2020-10-09 --
+-- Member/group limit override per-system
+
+alter table systems add column member_limit_override smallint default null;
+alter table systems add column group_limit_override smallint default null;
+
+-- Lowering global limit to 1000 in this commit, so increase it for systems already above that
+update systems s set member_count_override = 1500
+    where (select count(*) from members m where m.system = s.id) > 1000;
+
+update info set schema_version = 10;