Add search form for hidden content

Added search for comments and proposal_notifications, added tsv column
for search and rake tasks to update/create tsv vector.

app/models/proposal_notification.rb

@@ -1,6 +1,7 @@
 class ProposalNotification < ApplicationRecord
   include Graphqlable
   include Notifiable
+  include Searchable
 
   belongs_to :author, class_name: "User"
   belongs_to :proposal
@@ -55,6 +56,17 @@ class ProposalNotification < ApplicationRecord
     update(moderated: false)
   end
 
+  def searchable_values
+    {
+      title => "A",
+      body  => "B"
+    }
+  end
+
+  def self.search(terms)
+    pg_search(terms)
+  end
+
   private
 
     def set_author