FIX: Delete reviewables associated to posts automatically

Currently we don’t have an association between reviewables and posts.
This sometimes leads to inconsistencies in the DB as a post can have
been deleted but an associated reviewable is still present.

This patch addresses this issue simply by adding a new association to
the `Post` model and by using the `dependent: :destroy` option.

spec/models/post_spec.rb

@@ -3,9 +3,11 @@
 RSpec.describe Post do
   fab!(:coding_horror) { Fabricate(:coding_horror) }
 
+  let(:upload_path) { Discourse.store.upload_path }
+
   before { Oneboxer.stubs :onebox }
 
-  let(:upload_path) { Discourse.store.upload_path }
+  it { is_expected.to have_many(:reviewables).dependent(:destroy) }
 
   describe "#hidden_reasons" do
     context "when verifying enum sequence" do