allow searching for issues with empty topics

[SF#610787] points out that the classic template doesn't allow a way
to search for issues that don't have a topic. Priority, status, and
assigned do, so I've remedied the oversight by adding a new option to
Topic.

This closes [SF#610787].

Author: Justus Pendleton

templates/classic/html/issue.search.html

@@ -54,7 +54,10 @@
                 db_klass string:keyword;
                 db_content string:name;">
   <th i18n:translate="">Topic:</th>
-  <td metal:use-macro="search_select"></td>
+  <td metal:use-macro="search_select">
+    <option metal:fill-slot="extra_options" value="-1" i18n:translate=""
+            tal:attributes="selected python:value == '-1'">not selected</option>
+  </td>
   <td metal:use-macro="column_input"></td>
   <td metal:use-macro="sort_input"></td>
   <td metal:use-macro="group_input"></td>