db: add subject_id, subject_type columns to mod_actions.

The subject of the mod action is the post, user, or other object the modaction is for.

db/migrate/20220925045236_add_subject_to_mod_actions.rb

@@ -0,0 +1,9 @@
+class AddSubjectToModActions < ActiveRecord::Migration[7.0]
+  def change
+    add_column :mod_actions, :subject_type, :string, null: true
+    add_column :mod_actions, :subject_id, :integer, null: true
+
+    add_index :mod_actions, :subject_type
+    add_index :mod_actions, :subject_id
+  end
+end