[mod] isolation of botdetection from the limiter

This patch was inspired by the discussion around PR-2882 [2].  The goals of this
patch are:

1. Convert plugin searx.plugin.limiter to normal code [1]
2. isolation of botdetection from the limiter [2]
3. searx/{tools => botdetection}/config.py and drop searx.tools
4. in URL /config, 'limiter.enabled' is true only if the limiter is really
   enabled (Redis is available).

This patch moves all the code that belongs to botdetection into namespace
searx.botdetection and code that belongs to limiter is placed in namespace
searx.limiter.

Tthe limiter used to be a plugin at some point botdetection was added, it was
not a plugin.  The modularization of these two components was long overdue.
With the clear modularization, the documentation could then also be organized
according to the architecture.

[1] https://github.com/searxng/searxng/pull/2882
[2] https://github.com/searxng/searxng/pull/2882#issuecomment-1741716891

To test:

- check the app works without the limiter, check `/config`
- check the app works with the limiter and with the token, check `/config`
- make docs.live .. and read
  - http://0.0.0.0:8000/admin/searx.limiter.html
  - http://0.0.0.0:8000/src/searx.botdetection.html#botdetection

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

searx/botdetection/__init__.py

@@ -2,43 +2,22 @@
 # lint: pylint
 """.. _botdetection src:
 
-The :ref:`limiter <limiter src>` implements several methods to block bots:
-
-a. Analysis of the HTTP header in the request / can be easily bypassed.
-
-b. Block and pass lists in which IPs are listed / difficult to maintain, since
-   the IPs of bots are not all known and change over the time.
-
-c. Detection of bots based on the behavior of the requests and blocking and, if
-   necessary, unblocking of the IPs via a dynamically changeable IP block list.
-
-For dynamically changeable IP lists a Redis database is needed and for any kind
-of IP list the determination of the IP of the client is essential.  The IP of
-the client is determined via the X-Forwarded-For_ HTTP header
-
-.. _X-Forwarded-For:
-   https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-For
-
-X-Forwarded-For
-===============
-
-.. attention::
-
-   A correct setup of the HTTP request headers ``X-Forwarded-For`` and
-   ``X-Real-IP`` is essential to be able to assign a request to an IP correctly:
-
-   - `NGINX RequestHeader`_
-   - `Apache RequestHeader`_
-
-.. _NGINX RequestHeader:
-    https://docs.searxng.org/admin/installation-nginx.html#nginx-s-searxng-site
-.. _Apache RequestHeader:
-    https://docs.searxng.org/admin/installation-apache.html#apache-s-searxng-site
-
-.. autofunction:: searx.botdetection.get_real_ip
+Implementations used for bot detection.
 
 """
 
 from ._helpers import dump_request
 from ._helpers import get_real_ip
+from ._helpers import get_network
 from ._helpers import too_many_requests
+
+__all__ = ['dump_request', 'get_network', 'get_real_ip', 'too_many_requests']
+
+redis_client = None
+cfg = None
+
+
+def init(_cfg, _redis_client):
+    global redis_client, cfg  # pylint: disable=global-statement
+    redis_client = _redis_client
+    cfg = _cfg