add new demo symfony files

app/templates/default/_flash_messages.html.twig

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+{#
+   This is a template fragment designed to be included in other templates
+   See https://symfony.com/doc/current/templates.html#including-templates
+
+   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'
+#}
+
+{#
+   The check is needed to prevent starting the session when looking for "flash messages":
+   https://symfony.com/doc/current/session.html#avoid-starting-sessions-for-anonymous-users
+
+   TIP: With FOSHttpCache you can also adapt this to make it cache safe:
+   https://foshttpcachebundle.readthedocs.io/en/latest/features/helpers/flash-message.html
+#}
+{% if app.request.hasPreviousSession %}
+    <div class="messages">
+        {% 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">
+                    <button type="button" class="close" data-dismiss="alert" aria-label="{{ 'action.close'|trans }}">
+                        <span aria-hidden="true">&times;</span>
+                    </button>
+
+                    {{ message|trans }}
+                </div>
+            {% endfor %}
+        {% endfor %}
+    </div>
+{% endif %}

app/templates/default/homepage.html.twig

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+{% extends 'base.html.twig' %}
+
+{% block body_id 'homepage' %}
+
+{#
+    the homepage is a special page which displays neither a header nor a footer.
+    this is done with the 'trick' of defining empty Twig blocks without any content
+#}
+{% block header %}{% endblock %}
+{% block footer %}{% endblock %}
+
+{% block body %}
+    <div class="page-header">
+        <h1>{{ 'title.homepage'|trans|raw }}</h1>
+    </div>
+
+    <div class="row">
+        <div class="col-sm-6">
+            <div class="jumbotron">
+                <p>
+                    {{ 'help.browse_app'|trans|raw }}
+                </p>
+                <p>
+                    <a class="btn btn-primary btn-lg" href="{{ path('blog_index') }}">
+                        <i class="fa fa-users" aria-hidden="true"></i> {{ 'action.browse_app'|trans }}
+                    </a>
+                </p>
+            </div>
+        </div>
+
+        <div class="col-sm-6">
+            <div class="jumbotron">
+                <p>
+                    {{ 'help.browse_admin'|trans|raw }}
+                </p>
+                <p>
+                    <a class="btn btn-primary btn-lg" href="{{ path('admin_index') }}">
+                        <i class="fa fa-lock" aria-hidden="true"></i> {{ 'action.browse_admin'|trans }}
+                    </a>
+                </p>
+            </div>
+        </div>
+    </div>
+{% endblock %}