[feat] allow customization of logo and favicon

also removed searxng-wordmark from templates directory and build
files, because it is no longer used anywhere

searx/templates/simple/index.html

@@ -2,7 +2,7 @@
 {% from 'simple/icons.html' import icon_big %}
 {% block content %}
 <div class="index">
-    <div class="title"><h1>SearXNG</h1></div>
+    <div class="title{% if get_setting('brand.custom_files.logo', false) %} custom_logo{% endif %}"><h1>SearXNG</h1></div>
     {% include 'simple/simple_search.html' %}
 </div>
 {% endblock %}