Ask confirmation to delete content blocks from the edit page

We were already doing that when deleting content blocks from the index
page, and we also ask for confirmation in almost every page in the admin
section.

app/views/admin/site_customization/content_blocks/edit.html.erb

@@ -9,6 +9,7 @@
   @content_block,
   text: t("admin.site_customization.content_blocks.index.delete"),
   path: (@is_heading_content_block ? admin_site_customization_delete_heading_content_block_path(@content_block.id) : admin_site_customization_content_block_path(@content_block)),
+  confirm: true,
   method: :delete,
   class: "delete float-right"
 ) %>

spec/system/admin/site_customization/content_blocks_spec.rb

@@ -103,7 +103,9 @@ describe "Admin custom content blocks", :admin do
       block = create(:site_customization_content_block)
       visit edit_admin_site_customization_content_block_path(block)
 
+      accept_confirm("Are you sure? This action will delete \"#{block.name}\" and can't be undone.") do
         click_button "Delete block"
+      end
 
       expect(page).not_to have_content("#{block.name} (#{block.locale})")
       expect(page).not_to have_content(block.body)