Initial USWDS setup

[echappen]

Changes proposed in this pull request:

• Adds the USWDS package globally to the NextJS app (read the added README notes for more information)

Closes #50

Notes

• Example of a usa-banner is on the /design-guide page. I created this page as a place for testing out web components and other design elements as they're created.
• Ideally, we wouldn't need any static USWDS javascript assets if we convert USWDS components to React components. (For example, usa-banner could handle it's opening/closing with React state instead, which would eliminate the need to use uswds-init js.) Since this will be a gradual process, I've kept the USWDS javascript files in here as we transition to that.
• The way USWDS SASS is set up makes using NextJS style modules mostly incompatible, since every module would need to import the USWDS theme settings along with USWDS Core. For now, I suggest we add USWDS packages gradually to the global SASS entry point (styles.scss) until we see a need to have different entry points.

Security considerations

• Adds new npm dependencies: sass and @uswds/uswds
• Adds USWDS assets to a public folder. Since there's nothing scary in USWDS assets, this is fine.

[jduss4]

I'm so impressed, this would have taken me multiple weeks to get halfway working. Thank you for documenting the steps to update / add things so clearly, too, as I'm confident I will be relying heavily on it!

[echappen]

@hursey013 Any other comments/concerns before I merge?