Update JavaScript styleguide to use Standard

[tvararu]

We've been trying out standardjs on a few of our codebases:

• [RFC] Lint codebase using standard govuk-prototype-kit#224
• Lint codebase using standard govuk_elements#290
• Lint codebase using standard govuk_frontend_toolkit#334

It seems like it's been well received. I think we should make it part of our JavaScript styleguide. I see this as three parts:

• removing what is already defined in the Standard guidelines
• adding our reasoning behind using Standard, which can be pulled from the prototype kit
• updating any examples to adhere to the new style

[ChrisBAshton]

Looks like this was done in January 2017: https://github.com/alphagov/styleguides/commits/master/js.md