minor #571 Simplified the flash messages (javiereguiluz)

This PR was squashed before being merged into the master branch (closes #571).

Discussion
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Simplified the flash messages

As explained in https://symfony.com/blog/new-in-symfony-3-3-improved-flash-messages

Commits
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5c5a9ac Simplified the flash messages

Author: javiereguiluz

app/Resources/views/default/_flash_messages.html.twig

@@ -7,19 +7,17 @@
    '_flash_messages.html.twig' instead of 'flash_messages.html.twig'
 #}
 
-{% if app.session.started and app.session.flashBag.peekAll is not empty %}
-    <div class="messages">
-        {% for type, messages in app.session.flashBag.all %}
-            {% for message in messages %}
-                {# Bootstrap alert, see http://getbootstrap.com/components/#alerts #}
-                <div class="alert alert-dismissible alert-{{ type }}" role="alert">
-                    <button type="button" class="close" data-dismiss="alert" aria-label="Close">
-                        <span aria-hidden="true">&times;</span>
-                    </button>
+<div class="messages">
+    {% for type, messages in app.flashes %}
+        {% for message in messages %}
+            {# Bootstrap alert, see http://getbootstrap.com/components/#alerts #}
+            <div class="alert alert-dismissible alert-{{ type }}" role="alert">
+                <button type="button" class="close" data-dismiss="alert" aria-label="Close">
+                    <span aria-hidden="true">&times;</span>
+                </button>
 
-                    {{ message|trans }}
-                </div>
-            {% endfor %}
+                {{ message|trans }}
+            </div>
         {% endfor %}
-    </div>
-{% endif %}
+    {% endfor %}
+</div>