Add search method to polls

So far the method does not take questions nor answers into account.

This way we'll be able to search polls in the SDG Management section.

spec/lib/tasks/db_spec.rb

@@ -32,3 +32,22 @@ describe "rake db:load_sdg" do
     expect(SDG::Target.last.id).to eq target_id
   end
 end
+
+describe "rake db:calculate_tsv" do
+  before { Rake::Task["db:calculate_tsv"].reenable }
+
+  let :run_rake_task do
+    Rake.application.invoke_task("db:calculate_tsv")
+  end
+
+  it "calculates the tsvector for polls" do
+    poll = create(:poll)
+    poll.update_column(:tsv, nil)
+
+    expect(poll.reload.tsv).to be nil
+
+    run_rake_task
+
+    expect(poll.reload.tsv).not_to be nil
+  end
+end