use flask flash for notifications

templates/fragments/flash.html

@@ -0,0 +1,9 @@
+{% from "components/alert.html" import Alert %}
+
+{% with messages = get_flashed_messages(with_categories=true) %}
+  {% if messages %}
+    {% for category, message_config in messages %}
+      {{ Alert(message_config["title"], message=message_config.get("message"), level=category) }}
+    {% endfor %}
+  {% endif %}
+{% endwith %}