Link from the invoice number in expense index to its edit form

If there is no invoice number, then they can use the edit item from the
menu, but most expenses do have an invoice, thus this is easier for the
most usual case.

web/template/expenses/index.gohtml

@@ -82,7 +82,11 @@
                 <tr>
                     <td>{{ .InvoicerName }}</td>
                     <td>{{ .InvoiceDate|formatDate }}</td>
-                    <td>{{ .InvoiceNumber }}</td>
+                    <td>
+                        {{- if .InvoiceNumber -}}
+                            <a href="{{ companyURI "/expenses/"}}{{ .Slug }}">{{ .InvoiceNumber }}</a>
+                        {{- end -}}
+                    </td>
                     <td class="expense-status-{{ .Status }}">{{ .StatusLabel }}</td>
                     <td
                             data-hx-get="{{companyURI "/expenses/"}}{{ .Slug }}/tags/edit"