Do not call markdownify in alert partial

This causes issues for links as the method doesn't have
the right context to resolve them.
The content is already markdownified because of the `%` syntax of the shortcode
that calls it, and it has the right context.

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

layouts/partials/alert.html

@@ -16,5 +16,5 @@
 {{ $omit_title := .omit_title }}
 
 <div class="alert {{ $type }} {{ if $omit_title }}omit-title{{end}}" role="alert">
-{{ $content | markdownify }}
+{{ $content }}
 </div>