[SPIKE] Use GOV.UK Frontend for buttons

[tijmenb]

This uses the latest version of GOV.UK Frontend to render the buttons. GOV.UK Frontend is still in private beta, but this PR is mainly to see how we might consume the project (since it's The Future).

Some observations:

• I've added a package.json and added govuk_frontend package. We need to commit all of the package, but pulling in individual components works nicely. Images can be easily added to the asset pipeline 👌
• The new button is slightly larger than our button, but it looks okay.
  
• Our button component has more features than the component from GOV.UK Frontend. Namely we support a bottom margin and an "Info text" below the button. I've kept the existing classes for these, but it's quite weird to mix (govuk-button, gem-c-button).

Open issues

• The images don't work as they should yet. Although we're able to display the start button image, the resulting CSS links to the image directly. In Rails we normally use the image-url helper to refer to assets, so it will use the fingerprinting. I've tried overriding govuk-file-url() but that doesn't work. Fixed by Allow custom image and font URL helpers govuk-frontend#733
• The button component seems to have a fixed bottom margin, which causes the info text to be weird:
  
  Fixed by setting a bottom margin: 2315334
• The font is included by default, this might be a problem Fixed by Use new font loading strategy govuk-frontend#726

https://trello.com/c/L9Lhqngb