feat: delete command messages with pk;msg -delete

PluralKit.Core/Database/Migrations/17.sql

@@ -0,0 +1,8 @@
+-- schema version 17: 2021-09-26 --
+-- add channel_id to command message table
+
+alter table command_messages add column channel_id bigint;
+update command_messages set channel_id = 0;
+alter table command_messages alter column channel_id set not null;
+
+update info set schema_version = 17;