minor #1442 Show invisible alerts (fix #1440) (smnandre)

This PR was squashed before being merged into the main branch.

Discussion
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Show invisible alerts (fix #1440)

Add class "show" to avoid invisible alerts

See #1440

Commits
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5100c82 Show invisible alerts (fix #1440)

templates/default/_flash_messages.html.twig

@@ -19,7 +19,7 @@
         {% for type, messages in app.flashes %}
             {% for message in messages %}
                 {# Bootstrap alert, see https://getbootstrap.com/docs/3.4/components/#alerts #}
-                <div class="alert alert-dismissible alert-{{ type }} fade in" role="alert">
+                <div class="alert alert-dismissible alert-{{ type }} fade show" role="alert">
                     <button type="button" class="close" data-dismiss="alert" aria-label="{{ 'action.close'|trans }}">
                         <span aria-hidden="true">&times;</span>
                     </button>