Font scale tweaks & update

- [new] Add a new size to the textsans font scale (see https://github.com/guardian/guss-typography for the new font scale matrix)
- [refactor] Remove section-head (unused since the new nav) and simplify localnav title
- [refactor] Coding conventions / remify stuff
- [fix] Reordered fs- mixins in declarations where line-height was supposed to be overridden 

![ ](http://i.imgur.com/5nPH6gC.gif)