Don't allow to modify answer's images for started polls

Note that the `create` action doesn't create an image but updates an
answer instead. We're removing the references to `:create` in the
abilities since it isn't used.

In the future we might change the form to add an image to an answer
because it's been broken for ages since it shows all the attached
images.

app/controllers/admin/poll/questions/answers/images_controller.rb

@@ -2,6 +2,7 @@ class Admin::Poll::Questions::Answers::ImagesController < Admin::Poll::BaseContr
   include ImageAttributes
 
   load_and_authorize_resource :answer, class: "::Poll::Question::Answer"
+  load_and_authorize_resource only: [:destroy]
 
   def index
   end
@@ -11,6 +12,7 @@ class Admin::Poll::Questions::Answers::ImagesController < Admin::Poll::BaseContr
 
   def create
     @answer.attributes = images_params
+    authorize! :update, @answer
 
     if @answer.save
       redirect_to admin_answer_images_path(@answer),
@@ -21,7 +23,6 @@ class Admin::Poll::Questions::Answers::ImagesController < Admin::Poll::BaseContr
   end
 
   def destroy
-    @image = ::Image.find(params[:id])
     @image.destroy!
 
     respond_to do |format|