[mod] isolation of botdetection from SearXNG core

In PR-2894[1] we isolated botdetection from the limiter, this PR isolates the
botdetection from the SearXNG core code.

This PR also fixes the issue [2] that the ``server.public_instance`` option
needs to activate the limiter.

- [1] https://github.com/searxng/searxng/pull/2894
- [2] https://github.com/searxng/searxng/issues/2975

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

searx/templates/simple/base.html

@@ -17,7 +17,7 @@
   {% else %}
   <link rel="stylesheet" href="{{ url_for('static', filename='css/searxng.min.css') }}" type="text/css" media="screen" />
   {% endif %}
-  {% if get_setting('server.limiter') or get_setting('server.public_instance') %}
+  {% if get_setting('server.limiter') %}
   <link rel="stylesheet" href="{{ url_for('client_token', token=link_token) }}" type="text/css" />
   {% endif %}
   {% block styles %}{% endblock %}