Fix Commercial Component Targeting on Section Fronts

This fixes a bug where commercial component targeting wasn't working on section fronts.
Eg. http://www.theguardian.com/travel#commercial-component=books just shows generic bestsellers.

It also gets rid of a lot of redundant targeting.  We're only using keywords at the moment, except for travel offers which are specific to the travel section.

/cc @ironsidevsquincy  
what's the difference between `this.getKeywords` and https://github.com/guardian/frontend/blob/master/common/app/assets/javascripts/modules/adverts/query-string.js#L27