Update propsition navigation colours

Use a GOV.UK brand colour for the proposition navigation active colour
rather than a soon to be depricated department and policy turquoise.

Use black text for focus as it puts a yellow background on the links and
it doesn't hit contrast with white on yellow.
Move previously custom WH logic to align nav when there's no proposition title, expose it via a class name that apps can control
Re-order focus style so that the active nav link focus colour isn't overridden
Merge pull request #83 from alphagov/update-proposition-link-colours

Update propsition navigation colours