Initial USWDS Button component

[dgading]

We use buttons in many places in the data catalogs. We should get a React component that uses the USWDS styles and classes: https://designsystem.digital.gov/components/button/

It should take props for:

• [ ] Variant [default, secondary, accent, accent, warm, base, outline, unstyled]
• [ ] Disabled
• [ ] inverse bool
• [ ] big bool
• [ ] children (the button should take children so anything can be passed in from text to images)

Have documented props, default props, unit tests, and jest test.