Add favicon support.

* src/cuirass/http.scm (%file-white-list): Rename "logo.png" into "guix.png".
Add "icon.png" and "icon.svg".
* src/cuirass/templates.scm (html-page): Adapt it.
* src/static/images/logo.png: Rename it ...
* src/static/images/guix.png: ... into this file.
* src/static/images/icon.png: New file.
* src/static/images/icon.svg: New file.