Remove link around instance header, make logo optional
Remove link around instance header, make logo optional

* Remove the link around the instance header.
* do not include the img tag if do not pass a logo

alkadis/vcv,alkadis/vcv,phihag/adhocracy,alkadis/vcv,DanielNeugebauer/adhocracy,SysTheron/adhocracy,liqd/adhocracy,phihag/adhocracy,DanielNeugebauer/adhocracy,phihag/adhocracy,alkadis/vcv,liqd/adhocracy,SysTheron/adhocracy,phihag/adhocracy,DanielNeugebauer/adhocracy,alkadis/vcv,DanielNeugebauer/adhocracy,liqd/adhocracy,phihag/adhocracy,DanielNeugebauer/adhocracy,liqd/adhocracy,SysTheron/adhocracy