Use the vote action to vote featured proposals

The action and the views were almost identical, with the supports
progress and the HTML classes of the success message element being the
only exceptions; we can use CSS for the styles instead.

app/controllers/proposals_controller.rb

@@ -71,11 +71,6 @@ class ProposalsController < ApplicationController
   def retire_form
   end
 
-  def vote_featured
-    @follow = Follow.find_or_create_by!(user: current_user, followable: @proposal)
-    @proposal.register_vote(current_user, "yes")
-  end
-
   def summary
     @proposals = Proposal.for_summary
     @tag_cloud = tag_cloud