Further improve proposed pattern for Redirector

This patch further improves the proposed pattern for the Redirector
extension. In contrast to the old pattern, …

* … it will redirect the URL https://medium.com.
* … it will *not* redirect URLs with top-level domains like mediumXcom.
  (This point is purely theoretical, but it makes the regular expression
  more correct and consistent.)
* … it will *not* redirect URLs like https://link.medium.com/AXEtCilplkb
  which Scribe currently cannot handle. These are shortened URLs that
  users get when they use the Twitter button on Medium to share a post.

In order to implement the last point (not matching link.medium.com), the
pattern uses negative lookbehind. This feature of regular expressions is
supported by all recent browsers for which Redirector is available
(Firefox, Chrome, Edge, Opera)[^1], including the current version of
Firefox ESR (Extended Stability Release).

[^1]: https://caniuse.com/js-regexp-lookbehind

src/pages/home/index_page.cr

@@ -53,13 +53,13 @@ class Home::IndexPage < MainLayout
           end
           li do
             strong "Include pattern: "
-            code "^https?://(?:.*\\.)*medium.com/(.*)$"
+            code "^https?://(?:.*\\.)*(?<!link\\.)medium\\.com(/.*)?$"
           end
           li do
             strong "Redirect to: "
             code "https://"
             code app_domain
-            code "/$1"
+            code "$1"
           end
           li do
             strong "Pattern type: "