HTC longpress bug

Hyperlinks across all of gov.uk do not bring up the contextual menu when 'longpressed' on HTC handsets, as explained in this ticket:

https://govuk.zendesk.com/tickets/1390

This is an after-effect of our setting the ```-webkit-tap-highlight-color``` property to transparent (to solve this bug: www.yuiblog.com/blog/2010/10/01/quick-tip-customizing-the-mobile-safari-tap-highlight-color/). This makes the HTC default browser treat hyperlinks as normal text (which have a different longpress menu).

The colour applied to hyperlinks is the same as the focus colour we use anyway.