Fix @nik's mistake.

ticdesk/templates/ticdesk/base.html

@@ -22,7 +22,7 @@
 
   <div class="collapse navbar-collapse" id="navbar-main">
    <ul class="nav navbar-nav ml-auto">
-   {% if user.is_member %}
+   {% if current_person.is_member %}
     <form class="form-inline" action="{% url "ticdesk_org:person" %}" method="GET">
      <input class="form-control mr-sm-2" type="search" placeholder="Benutzername" aria-label="Search" name="uid" />
     </form>