tipue_search search improvement

Support Flex search, that is located in a sub directory, like https://FQDN/blog.

templates/base.html

@@ -175,7 +175,7 @@
       {% if SITESUBTITLE %}<p>{{ SITESUBTITLE }}</p>{% endif %}
 
       {% if PLUGINS and 'tipue_search' in PLUGINS %}
-        <form class="navbar-search" action="/search.html" role="search">
+        <form class="navbar-search" action="{{ SITEURL }}/search.html" role="search">
           <input type="text" name="q" id="tipue_search_input" placeholder="{{ _('Search...') }}">
         </form>
       {% endif %}