[mod] Revision of the favicon solution

All favicons implementations have been documented and moved to the Python
package:

    searx.favicons

There is a configuration (based on Pydantic) for the favicons and all its
components:

    searx.favicons.config

A solution for caching favicons has been implemented:

    searx.favicon.cache

If the favicon is already in the cache, the returned URL is a data URL [1]
(something like `data:image/png;base64,...`).  By generating a data url from
the FaviconCache, additional HTTP roundtripps via the favicon_proxy are saved:

    favicons.proxy.favicon_url

The favicon proxy service now sets a HTTP header "Cache-Control: max-age=...":

    favicons.proxy.favicon_proxy

The resolvers now also provide the mime type (data, mime):

    searx.favicon.resolvers

[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URLs

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

searx/preferences.py

@@ -13,7 +13,7 @@ from collections import OrderedDict
 import flask
 import babel
 
-from searx import settings, autocomplete, favicon_resolver
+from searx import settings, autocomplete, favicons
 from searx.enginelib import Engine
 from searx.plugins import Plugin
 from searx.locales import LOCALE_NAMES
@@ -409,7 +409,7 @@ class Preferences:
             'favicon_resolver': EnumStringSetting(
                 settings['search']['favicon_resolver'],
                 locked=is_locked('favicon_resolver'),
-                choices=list(favicon_resolver.backends.keys()) + ['']
+                choices=list(favicons.proxy.CFG.resolver_map.keys()) + ['']
             ),
             'image_proxy': BooleanSetting(
                 settings['server']['image_proxy'],