Implement custom focus ring

[ezhangy]

Implement the NJWDS custom focus ring. This focus ring differs from the USWDS version in order to improve contrast on dark backgrounds. As can be seen in the image below, the custom focus ring has:

• An outer blue outline and inner white outline
• Some margin between the inner white outline and the element so that the background color can show through.

The CSS to implement this is not straightforward, so leaving as an issue for now.