Check favicon sizes and formats

depends on https://github.com/cargomedia/CM/pull/1421

@christopheschwyzer @mariansollmann @dlondero 
Should we check our usage of *favicon* and other *splash*, *touch* or *tile* images before creating a lot of these for the new sites?

Summary: https://github.com/audreyr/favicon-cheat-sheet
PNG Favicon: http://www.jonathantneal.com/blog/understand-the-favicon/