Design overhaul to use US Web Design Standards

[konklone]

This pull request is a long-overdue visual overhaul for https.cio.gov, which:

• Uses the US Web Design Standards
• Matches the theme of cyber.dhs.gov and itmodernization.cio.gov
• Has a sticky sidebar
• Works way better on mobile

Main page:

And a shot of a scrolled-down page, with sticky header as well as the rendering for <code> and <pre> sections:

cc @hmft @gbinal

[konklone]

I also removed the Travis build file, as we no longer use Travis and I'm not yet certain how to replicate the auto-deploy setup in Circle. I think auto-deployment is likely best done by moving to Federalist, anyhow (the site is still, for now, deployed directly via cloud.gov).

[rustypickens]

Looks great, Eric! Very well organized and easy to access. Great work.

[gbinal]

Looks great! Glad to utilize the design standards.

Let's do this.

[konklone]

Thanks, all! Also, another measure of the USWDS' success:

[h-m-f-t]

Very nice! 🎉