js/helpers: add $.hasAttr

Need this for another fix and there is no such jQuery function. As this
seems to also be inconsistant across browsers I thought this would
definitively be worth a custom jQuery plugin.
js/ui: check whether a link tag has a type

The new favicon didn't have such, resulting in JS error when reloading
CSS at runtime.
favicon: add link type

The favicon link tag was missing a type attribute.