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.

db/migrate/20190307113726_add_tsvector_to_comments_and_proposal_notifications.rb

@@ -0,0 +1,9 @@
+class AddTsvectorToCommentsAndProposalNotifications < ActiveRecord::Migration[5.2]
+  def change
+    add_column :comments, :tsv, :tsvector
+    add_index :comments, :tsv, using: "gin"
+
+    add_column :proposal_notifications, :tsv, :tsvector
+    add_index :proposal_notifications, :tsv, using: "gin"
+  end
+end