preferences: Set autocomplete=off for form

Otherwise you can change the value of a select,
refresh the page and the preferences stay changed,
leaving the wrong impression that they were saved.

searx/templates/oscar/preferences.html

@@ -94,7 +94,7 @@
 <div>
 
     <h1>{{ _('Preferences') }}</h1>
-    <form method="post" action="{{ url_for('preferences') }}" id="search_form">
+    <form method="post" action="{{ url_for('preferences') }}" id="search_form" autocomplete="off">
 
         <!-- Nav tabs -->
         <ul class="nav nav-tabs nav-justified hide_if_nojs" role="tablist">