Fix Ad Targeting on Commercial Microsite Fronts

There's a problem where Ad Ops are having to target different keywords for microsite fronts and content pages.  For example, in ad calls [this front](http://www.theguardian.com/sustainable-business/grundfos-partner-zone) passes *grundfos-partner-zone* as its keyword parameter whereas [this content page](http://www.theguardian.com/sustainable-business/grundfos-partner-zone/2014/oct/06/saving-co2-desalination-water-italy-aeolian-islands) in the same section passes *sustainable-business-grundfos-partner-zone* as its section keyword parameter. 

This change solves the problem by giving section fronts a keyword ID that corresponds with the section keyword ID for pages within the section.  Then ad calls target the keyword in the same way as any other keyword.