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.

app/models/post.rb

@@ -61,6 +61,7 @@ class Post < ActiveRecord::Base
   belongs_to :image_upload, class_name: "Upload"
 
   has_many :post_hotlinked_media, dependent: :destroy, class_name: "PostHotlinkedMedia"
+  has_many :reviewables, as: :target, dependent: :destroy
 
   validates_with PostValidator, unless: :skip_validation