[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/src/searx.favicons.rst

@@ -0,0 +1,48 @@
+.. _favicons:
+
+========
+Favicons
+========
+
+.. contents::
+   :depth: 2
+   :local:
+   :backlinks: entry
+
+.. automodule:: searx.favicons
+   :members:
+
+.. _favicons.config:
+
+Favicons Config
+===============
+
+.. automodule:: searx.favicons.config
+   :members:
+
+.. _favicons.proxy:
+
+Favicons Proxy
+==============
+
+.. automodule:: searx.favicons.proxy
+   :members:
+
+.. _favicons.resolver:
+
+Favicons Resolver
+=================
+
+.. automodule:: searx.favicons.resolvers
+   :members:
+
+.. _favicons.cache:
+
+Favicons Cache
+==============
+
+.. automodule:: searx.favicons.cache
+   :members:
+
+
+