Add consistency to back_links

In some cases (e.g. after editing or creating a resource
with errors) the default back_link did not redirect to the
expected page.

Now we force the back_links to the index pages, so we
always get the desired redirect.

spec/system/admin/poll/questions/answers/answers_spec.rb

@@ -7,6 +7,8 @@ describe "Answers", :admin do
     visit admin_question_path(question)
     click_link "Add answer"
 
+    expect(page).to have_link "Go back", href: admin_question_path(question)
+
     fill_in "Answer", with: "The answer is always 42"
     fill_in_ckeditor "Description", with: "The Hitchhiker's Guide To The Universe"
 
@@ -39,6 +41,8 @@ describe "Answers", :admin do
     visit admin_question_path(question)
     within("tr", text: "Answer title") { click_link "Edit" }
 
+    expect(page).to have_link "Go back", href: admin_question_path(question)
+
     fill_in "Answer", with: "New title"
     click_button "Save"