Update header

- updates pop over style with group headings, bigger anchor arrows etc
- control icons adopt hover behaviour of message actions
- moves ‘more’ menu to left-aligned ellipsis icon below desktop bp, adds options to it too
- all header text links underline on hover - fixes #5425
- brings identity header into line with main header - fixes #5598

![screen shot 2014-08-19 at 15 16 35](https://cloud.githubusercontent.com/assets/867233/3967319/1a2b6f00-27ac-11e4-9f1d-4c7af5df4101.png)

Remify (until we decide if we keep using `px_to_rem` or not