feat: add Sentry User Feedback widget to backoffice (#178)

Install @sentry/browser via AssetMapper and initialize the feedback
integration on all backoffice pages. The CDN loader script is moved to
an overridable block so backoffice pages use the npm SDK without
double-initialising Sentry.

Sentry is initialised only when a DSN is configured (SENTRY_DSN env
var). When a user is logged in their email is pre-filled in the feedback
form via Sentry.setUser(); both name and email remain optional so
anonymous submissions are supported by simply leaving those fields empty.

templates/backoffice/base.html.twig

@@ -1,5 +1,10 @@
 {% extends 'base.html.twig' %}
 {% block importmap %}{{ importmap('backoffice') }}{% endblock %}
+{% block sentry_loader %}{% endblock %}
+{% block head_extra %}
+    <meta name="sentry-dsn" content="{{ sentry_dsn }}">
+    <meta name="user-email" content="{{ app.user ? app.user.userIdentifier : '' }}">
+{% endblock %}
 {% block title %}Tijd voor de test | {% endblock %}
 {% block nav %}{{ include('backoffice/nav.html.twig') }}{% endblock %}