Fix session starting issue in symfony-demo on flash messages

Use of flash messages triggers session start, which as of last year
triggers Symfony to mark response as private.

Fix the issue by checking if there is a session first as adviced on:
https://symfony.com/doc/current/session/avoid_session_start.html

Author: André R

templates/default/_flash_messages.html.twig

@@ -5,19 +5,27 @@
    A common practice to better distinguish between templates and fragments is to
    prefix fragments with an underscore. That's why this template is called
    '_flash_messages.html.twig' instead of 'flash_messages.html.twig'
+
+   We check if we have session before reading flashes as it otherwise triggers session start:
+   https://symfony.com/doc/current/session/avoid_session_start.html
+
+   TIP: With FOSHttpCache you can also adapt this to make it cache safe:
+   https://foshttpcachebundle.readthedocs.io/en/latest/features/helpers/flash-message.html
 #}
 
-<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 }} fade in" role="alert">
-                <button type="button" class="close" data-dismiss="alert" aria-label="Close">
-                    <span aria-hidden="true">&times;</span>
-                </button>
+{% if app.request.hasPreviousSession %}
+    <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 }} fade in" role="alert">
+                    <button type="button" class="close" data-dismiss="alert" aria-label="Close">
+                        <span aria-hidden="true">&times;</span>
+                    </button>
 
-                {{ message|trans }}
-            </div>
+                    {{ message|trans }}
+                </div>
+            {% endfor %}
         {% endfor %}
-    {% endfor %}
-</div>
+    </div>
+{% endif %}