Don't delegate preview methods to controller.

The controller will try to load the Election from the params but when
rendering an election that's the single one in a voting there are no
params. Instead, it's better to use the model in the cell.

decidim-elections/app/cells/decidim/elections/election_vote_cta_cell.rb

@@ -5,15 +5,19 @@ module Elections
     # This cell renders the results
     # for a given instance of an Election
     class ElectionVoteCtaCell < Decidim::ViewModel
+      include Decidim::Elections::HasVoteFlow
+
       delegate :current_user,
                :current_participatory_space,
-               :preview_mode?,
-               :can_preview?,
-               :vote_flow,
                to: :controller
 
       private
 
+      # This is needed by HasVoteFlow
+      def election
+        model
+      end
+
       def last_vote
         @last_vote ||= Decidim::Elections::Votes::LastVoteForVoter.for(model, vote_flow.voter_id) if vote_flow.has_voter?
       end