Introduce USWDS theming and upgrade to USWDS 3.8.

[sanason]

This PR does 2 things:

1. Updates the version of USWDS we are using from 3.6.1 to 3.8. This will give us access to a new sticky table header feature which we can use for https://trello.com/c/7f3i1ylK/79-when-a-table-is-longer-than-viewport-table-header-row-stays-visible-on-scroll.
2. Adopts a somewhat more formal procedure for upgrading and customizing USWDS in our project (documented in the README).

Number 2 is just a proposal, from a brand new member of the team who has no context on why things are done the way they are done so, reviewers, feel free to criticize. That said, I think the new procedure offers these benefits:

• Clearer documentation of which USWDS version we are on (since it's now recorded in package.json).
• Single automated command that ensures that all updated assets are copied to the right place (gulp updateUswds).
• Follows the "approved" approach for customizing USWDS (by adjusting the settings provided by the framework) which makes our customizations less likely to break when USWDS is updated. (Note that this PR only takes one tiny step in this direction.)

Testing

Did the upgrade to 3.8 break anything in the Touchpoints app? Did the upgrade to 3.8 break anything in the display of questionnaire forms?

No changes that I can see. The only breaking change listed (in 3.8) concerns the class usa-layout-docs__sidenav which, as far as I can tell, we aren't really using.

Do our tables look the same after using the theme-table-border-color setting to customize their appearance?

Yes, borders look the same.

[sanason]

Follows the "approved" approach for customizing USWDS (by adjusting the settings provided by the framework) which makes our customizations less likely to break when USWDS is updated.

Here's an example of customizations being broken: Originally, I upgraded to 3.8 simply by copying over the new versions of uswds.js and uswds.css. Then I added the new class usa-table--sticky-header to the list-of-forms table and got this result:

The new class added a border to the table and, since we weren't setting theme-table-border-color, we ended up with a black border.