[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/webapp.py

@@ -57,7 +57,9 @@ from searx import (
 )
 
 from searx import infopage
-from searx.botdetection import limiter
+from searx import limiter
+from searx.botdetection import link_token
+
 from searx.data import ENGINE_DESCRIPTIONS
 from searx.results import Timing
 from searx.settings_defaults import OUTPUT_FORMATS
@@ -94,7 +96,6 @@ from searx.utils import (
 from searx.version import VERSION_STRING, GIT_URL, GIT_BRANCH
 from searx.query import RawTextQuery
 from searx.plugins import Plugin, plugins, initialize as plugin_initialize
-from searx.botdetection import link_token
 from searx.plugins.oa_doi_rewrite import get_doi_resolver
 from searx.preferences import (
     Preferences,
@@ -1288,7 +1289,7 @@ def config():
                 'DOCS_URL': get_setting('brand.docs_url'),
             },
             'limiter': {
-                'enabled': settings['server']['limiter'],
+                'enabled': limiter.is_installed(),
                 'botdetection.ip_limit.link_token': _limiter_cfg.get('botdetection.ip_limit.link_token'),
                 'botdetection.ip_lists.pass_searxng_org': _limiter_cfg.get('botdetection.ip_lists.pass_searxng_org'),
             },
@@ -1322,6 +1323,7 @@ if not werkzeug_reloader or (werkzeug_reloader and os.environ.get("WERKZEUG_RUN_
     redis_initialize()
     plugin_initialize(app)
     search_initialize(enable_checker=True, check_network=True, enable_metrics=settings['general']['enable_metrics'])
+    limiter.initialize(app, settings)
 
 
 def run():