Extract documents action from Answer controller

This way we have a controller just to manage
Poll::Question::Answer related documents in the
same way we have for videos and images.

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

@@ -0,0 +1,29 @@
+class Admin::Poll::Questions::Answers::DocumentsController < Admin::Poll::BaseController
+  include DocumentAttributes
+
+  load_and_authorize_resource :answer, class: "::Poll::Question::Answer"
+
+  def index
+  end
+
+  def create
+    @answer.attributes = documents_params
+
+    if @answer.save
+      redirect_to admin_answer_documents_path(@answer),
+        notice: t("admin.documents.create.success_notice")
+    else
+      render :new
+    end
+  end
+
+  private
+
+    def documents_params
+      params.require(:poll_question_answer).permit(allowed_params)
+    end
+
+    def allowed_params
+      [documents_attributes: document_attributes]
+    end
+end