Remove normalize

Normalize isn't needed for the header footer and shouldn't be mandated
on projects.
Include header and footer explicitly

There is no need for an extra file it just makes it more complicated to
follow.
Remove duplication as this is set twice
Standardise imports and partials

- Partial filenames should start with an underscore
- Extensions aren't needed in include calls
- Underscores aren't needed when including partials
Merge pull request #15 from alphagov/css-clean

CSS clean