Add group and system colors

PluralKit.Core/Database/Migrations/13.sql

@@ -0,0 +1,7 @@
+-- SCHEMA VERSION 13: 2021-03-28 --
+-- Add system and group colors --
+
+alter table systems add column color char(6);
+alter table groups add column color char(6);
+
+update info set schema_version = 13;