Add super basic group model/command

PluralKit.Core/Database/Functions/functions.sql

@@ -111,4 +111,14 @@ begin
         if not exists (select 1 from members where hid = new_hid) then return new_hid; end if;
     end loop;
 end
+$$ language plpgsql volatile;
+
+create function find_free_group_hid() returns char(5) as $$
+declare new_hid char(5);
+begin
+    loop
+        new_hid := generate_hid();
+        if not exists (select 1 from groups where hid = new_hid) then return new_hid; end if;
+    end loop;
+end
 $$ language plpgsql volatile;