addresses review feedback

Corrects 876bfa2's removal of these decorative IMGs without replacing
their semantic content.

thread: #6041 (comment)
squash/fixup: 876bfa2

securedrop/source_templates/flashed.html

@@ -4,7 +4,7 @@
   <ul>
   {% for category, message in messages %}
     {% if category != 'banner-warning' %}
-      <li class="flash {{ category }}">
+      <li class="flash {{ category }}" aria-label="{{ gettext(category) }}">
         {{ message }}
       </li>
     {% endif %}