[enh] add SVG favicon href="favicon.svg" type="image/svg+xml"

Add link::

    <link rel="icon" href="favicon.png" sizes="any">
    <link rel="icon" href="favicon.svg" type="image/svg+xml">

Modern browsers should request::

    INFO  werkzeug   : 127.0.0.1 - - [28/Nov/2021 17:03:07] "GET /static/themes/simple/img/favicon.svg HTTP/1.1" 200 -

[1] https://github.com/audreyfeldroy/favicon-cheat-sheet#svg-file
[2] https://dev.to/masakudamatsu/favicon-nightmare-how-to-maintain-sanity-3al7



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