[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>

tests/unit/test_preferences.py

@@ -2,6 +2,7 @@
 # pylint: disable=missing-module-docstring, invalid-name
 
 from tests import SearxTestCase
+from searx import favicons
 from searx.locales import locales_initialize
 from searx.preferences import (
     EnumStringSetting,
@@ -14,6 +15,7 @@ from searx.preferences import (
 from searx.plugins import Plugin
 
 locales_initialize()
+favicons.init()
 
 
 class PluginStub(Plugin):  # pylint: disable=missing-class-docstring, too-few-public-methods