Don't rely on specific classes for subsections.

By genericising the cucumber feature to just 'click on <x>' rather than
'click on the <x> subsection' we don't lose any clarity, but we remove
reliance on specific classes (i.e. the previous 'within' block to detect
where on the page to click)
Merge pull request #50 from alphagov/dont_rely_on_the_dom_so_much

Don't rely on specific classes for subsections.