[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>

docs/conf.py

@@ -127,6 +127,7 @@ extensions = [
     "sphinx_tabs.tabs", # https://github.com/djungelorm/sphinx-tabs
     'myst_parser',  # https://www.sphinx-doc.org/en/master/usage/markdown.html
     'notfound.extension',  # https://github.com/readthedocs/sphinx-notfound-page
+    'sphinxcontrib.autodoc_pydantic',  # https://github.com/mansenfranzen/autodoc_pydantic
 ]
 
 autodoc_default_options = {