Split custom filter migration script using the post-deployment migration system

There were some concerns with the custom filter migration script dropping a table,
thus making it unsafe to run in a zero-downtime setting. Upstream introduced
a way to run migrations after deployment, so revisit the old migration script to
make use of this.

db/post_migrate/20180813160548_post_migrate_filters.rb

@@ -0,0 +1,11 @@
+class PostMigrateFilters < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def up
+    drop_table :glitch_keyword_mutes if table_exists? :glitch_keyword_mutes
+  end
+
+  def down
+  end
+end
+