Fix the skip to content link

[jimmynotjim]

The link was once messing up the layout, but had been hidden entirely at some point. This change duplicates the skip link styles from cfgov-refresh.

Fixes #270

Additions

• Adds skip link styles from cfgov-refresh.

Testing

• Run grunt build
• Navigate to http://localhost:4000/ and press tab, you should see the button

Preview this PR without the whitespace changes

Screenshots

Checklist

• [x] Changes are limited to a single goal (no scope creep)
• [x] Code can be automatically merged (no conflicts)
• [x] Code follows the standards laid out in the front end playbook
• [x] Passes all existing automated tests
• [x] New functions include new tests
• [x] New functions are documented (with a description, list of inputs, and expected output)
• [x] Placeholder code is flagged
• [x] Visually tested in supported browsers and devices
• [x] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)