Sequence depends on click context

If an element has a `data-link-context` attribute, this'll specify the endpoint for retrieving the swipe sequence for any child links of that element. 

Allows overriding of the default behaviour of using `/front-trails/{section name}` with the section extracted from link url.

Note: the clickSpec object returned by clickStream now contains the original event, allowing any clickStream listener to do preventDefault() etc.