Implement Global Search capability

Splitting off from #192 (closed)

Currently 'socialorsearch' controls wether to display the search bar. This queries the course search.

For sites which implement a search engine for Global Search we want to be able to switch that box over to query the Global Search rather than course search.

Not sure wether that should be a 3rd option in that dropdown or a new setting.

But when we flick it over I think we also need to make the search bar in the mobile /small screen size query against global search .