[web-client] simple theme: refactor jinja_svg_catalog and cleanup icon names

This patch implements the template generation of the template:

  searx/templates/simple/icons.html

by the way the icon set (the icon names) has been normalized:

  film-outline   --> film
  magnet-outline --> magnet
  ..
  warning        --> alert

Some missing (categorie) icons had been added.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/templates/simple/stats.html

@@ -7,7 +7,7 @@
     {% if selected_engine_name %}
         {{ column_name }}
     {% elif column_order==sort_order %}
-        {{ column_name }} {{ icon_big('arrow-dropdown') }}
+        {{ icon_big('navigate-down') }} {{ column_name }}
     {% else %}
         <a href="{{ url_for('stats', sort=column_order) }}">{{ column_name }}</a>
     {% endif %}