Make link colours consistent with rest of gov.uk

As discussed with @russellthorn and @benterrett, it seems like a good idea to start being consistent. (Sorry @edds, orange is not hot.)

This is a work in progress. The main regression is that all links across the site now have underlines. In some cases this is a good thing (e.g. [what's the link here?](https://f.cloud.github.com/assets/40906/879584/bdd992aa-f92a-11e2-9ff6-137731ad0260.png)) but makes things a bit cluttered [in other cases](https://f.cloud.github.com/assets/40906/879601/0847dd4c-f92b-11e2-8f8e-8125b0bb8900.png).

I'll just leave this here so some more eyes can look over it, in particular designery ones.

Discuss!