Configure sentry for admin JS bundle

So we get reports about JS errors in the admin bundle.

This is particularily important since we introduced a bug in the past
that didn't confirm the users delete form which could have led to
deleting users and all their data without a confirmation dialog.

h/static/scripts/admin-site.js

+'use strict';
+
+// configure error reporting
+var settings = require('./settings')(document);
+if (settings.raven) {
+  require('./raven').init(settings.raven);
+}
+
 window.$ = window.jQuery = require('jquery');
 require('bootstrap');