[fix] don't show a category if there is no active engine in

When deactivate all the engines of a category, this category should disappeare.
This feature has been lost in commit 8e9ad1cc.

For better readability, webapp.get_enabled_categories() has been rewritten with
identical functionality.

Related:

- https://github.com/searxng/searxng/issues/1020
- https://github.com/searxng/searxng/issues/1604

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

searx/templates/simple/categories.html

@@ -13,7 +13,7 @@
 }  -%}
 <div id="categories" class="search_categories">{{- '' -}}
     <div id="categories_container">
-        {%- for category in categories_as_tabs -%}
+        {%- for category in categories -%}
         <div class="category"><input type="checkbox" id="checkbox_{{ category|replace(' ', '_') }}" name="category_{{ category }}"{% if category in selected_categories %} checked="checked"{% endif %}/>
             <label for="checkbox_{{ category|replace(' ', '_') }}" class="tooltips">
                 {{- icon_big(category_icons[category]) if category in category_icons  else icon_big('globe-outline') -}}