Add disabling reaction pings

PluralKit.Core/Migrations/5.sql

@@ -1,3 +1,3 @@
--- SCHEMA VERSION 4: 2020-02-14
+-- SCHEMA VERSION 5: 2020-02-14
 alter table servers add column log_cleanup_enabled bool not null default false;
 update info set schema_version = 5;

PluralKit.Core/Migrations/6.sql

@@ -0,0 +1,3 @@
+-- SCHEMA VERSION 6: 2020-03-21
+alter table systems add column pings bool not null default true;
+update info set schema_version = 6;