Make proposition menu more legible

I propose we make the proposition header white at all times as not all
propositions or services will have colours that pass accessibility 
contrast tests against black. I also propose we increase the size of 
the links to 19px as they are very small and difficult to read at 
present (and may also present accessibility challenges).
Move margin from logo to search bar

Stops logo jumping around when using different versions of the header
Embolden proposition navigation
Align proposition menu to baseline grid
Merge pull request #226 from alphagov/embiggen-prop-links

Make proposition menu more legible