Fix back navigation from activity page to cards (#2727)

Three changes needed to support navigating back from a card to the
activity page:

- Add root_path to the prefer_referrer allowlist on the card show page
- Switch event links from HTML target="_top" to data-turbo-frame="_top"
  so Turbo handles the navigation and turbo:before-visit fires to save
  the referrer
- Normalize trailing slashes in the referrer path comparison so
  /account_id and /account_id/ both match

ref: https://app.fizzy.do/5986089/cards/2390

test/system/back_link_navigation_test.rb

@@ -26,6 +26,30 @@ class BackLinkNavigationTest < ApplicationSystemTestCase
     assert_current_path filter_url, ignore_query: false
   end
 
+  test "card back link returns to activity page when navigating from it" do
+    sign_in_as(users(:david))
+
+    assert_text "Layout is broken"
+    click_on "Layout is broken"
+
+    assert_selector "a.btn--back strong", text: "Back to Home"
+    find("a.btn--back").click
+    assert_current_path root_path
+  end
+
+  test "card back link returns to activity page when navigating from it without trailing slash" do
+    sign_in_as(users(:david))
+
+    activity_url_without_trailing_slash = root_url.chomp("/")
+    visit activity_url_without_trailing_slash
+    assert_text "Layout is broken"
+    click_on "Layout is broken"
+
+    assert_selector "a.btn--back strong", text: "Back to Home"
+    find("a.btn--back").click
+    assert_current_path activity_url_without_trailing_slash
+  end
+
   test "card back link is not rewritten when navigating from a non-filter page" do
     sign_in_as(users(:david))