use flask-bootstrap util to display flashed messages

templates/base.html

@@ -1,16 +1,11 @@
 {% extends "bootstrap/base.html" %}
+{% import "bootstrap/utils.html" as utils %}
 
 {% block title %}Prolefeeder{% endblock %}
 {% block content %}
 <div class="container">
 
-{% with messages = get_flashed_messages() %}
-{% if messages %}
-    {% for message in messages %}
-    <div class="alert alert-info" role="alert">{{ message }}</div>
-    {% endfor %}
-{% endif %}
-{% endwith %}
+{{ utils.flashed_messages() }}
 
 {% block app_content %}
 {% endblock %}