automatize favicon creation

[csae8092]

to make future @laurauntner live easier provide a script make_favicons.py based on favicons which

• creates favicons from a given image
• creates the matching html snippet

ideally the script is triggered on build, so we don't need to keep the favicons in the git-repo

also try to refactor xslt/partials/html_head.xsl by creating a xslt/partials/html_favicon.xsl (which is then recreated by running make_favicons.py)