Fix Categories loophole

[prettyalana]

Because of how my Category model is set up

  has_many :listings, class_name: "Listing", foreign_key: "category_id", dependent: :destroy
  validates :name, uniqueness: true
end

and the foreign key constraint between the categories and the listings due in part to Listings belongs_to :category, class_name: "Category" relationship. When a category that is associated with listings is deleted, the listing also gets deleted leading to messages also becoming obsolete.