add checkin button (#25)

Reviewed-on: #25

templates/admin/checked_in.html.twig

@@ -0,0 +1,7 @@
+{% if not field.value %}
+    <form action="{{ path('admin_checkin', {'entityId': entityId(field)}) }}" method="post">
+        <button type="submit">Einchecken</button>
+    </form>
+{% else %}
+<button disabled>Eingecheckt</button>
+{% endif %}