Float to a 2 col homepage header on big screens

The 'Get a dashboard' part of the homepage should only appear
on the right hand side on big screens.

On small screens, it should just flow underneath the header.
Remove alpha tag from proposition link

This needs to be done because the marvellous @henryhadlow has added
an underline to this link on hover, which will look funny when we
upgrade to it.
Merge pull request #584 from alphagov/fix-homepage-header-small-screen

Fix homepage header on small screens