Enable tab focus outline on truss.works

[xtine]

Issue 5: Tab focus is missing
The focused state when tabbing isn’t visible anywhere on the site, making it difficult for keyboard users to orient themselves.
Doc with all documented accessibility issues:
https://docs.google.com/document/d/1upM9ue4UAuJwIBbZ6fRdjGKmzs-odcWxDQfYftIrHyw/edit?usp=sharing

PT IMPORTED STORY: https://www.pivotaltracker.com/story/show/171138579

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Getting tab focus to work within the constraints of our Squarespace theme proves difficult without JavaScript code injection (because we cannot easily edit the underlying HTML/CSS) which I would not recommend in this circumstance.