feat: cache Discord DM channels in database

PluralKit.Core/Database/Migrations/26.sql

@@ -0,0 +1,12 @@
+-- schema version 26
+-- cache Discord DM channels in the database
+
+alter table accounts alter column system drop not null;
+alter table accounts drop constraint accounts_system_fkey;
+alter table accounts
+    add constraint accounts_system_fkey
+    foreign key (system) references systems(id) on delete set null;
+
+alter table accounts add column dm_channel bigint;
+
+update info set schema_version = 26;