Correct handling of spaces in paperwork filenames

Also normalises display of Invoice IDs. Partial fix for #391.

RIGS/templates/invoice_list.html

@@ -23,7 +23,7 @@
                 <tbody>
                 {% for invoice in invoice_list %}
                     <tr class="table-{% if invoice.is_closed %}success{% else %}warning{% endif %}">
-                        <th scope="row">{{ invoice.pk }}<br>
+                        <th scope="row">{{ invoice.display_id }}<br>
                         <span class="text-muted">{% if invoice.void %}(VOID){% elif invoice.is_closed %}(PAID){% else %}(O/S){% endif %}</span></th>
                         <td><a href="{% url 'event_detail' invoice.event.pk %}">N{{ invoice.event.pk|stringformat:"05d" }}</a>: {{ invoice.event.name }} <br>
                             <span class="text-muted">{{ invoice.event.get_status_display }}{% if not invoice.event.mic %}, No MIC{% endif %}