searchable: refactor searchable_includes.

Pass searchable associations directly to search_attributes instead of
defining them separately in searchable_includes.

app/models/dmail.rb

@@ -98,7 +98,7 @@ class Dmail < ApplicationRecord
     end
 
     def search(params)
-      q = search_attributes(params, :id, :created_at, :updated_at, :is_read, :is_deleted, :title, :body)
+      q = search_attributes(params, :id, :created_at, :updated_at, :is_read, :is_deleted, :title, :body, :to, :from)
       q = q.text_attribute_matches(:title, params[:title_matches])
       q = q.text_attribute_matches(:body, params[:message_matches], index_column: :message_index)
 
@@ -180,10 +180,6 @@ class Dmail < ApplicationRecord
     key ? "dmail ##{id}/#{self.key}" : "dmail ##{id}"
   end
 
-  def self.searchable_includes
-    [:to, :from]
-  end
-
   def self.available_includes
     [:owner, :to, :from]
   end