:focus-visible polyfill

[ffoodd]

Fixes #139

• PR on Bootstrap to contribute back: won't be merged :/
• focus-visible polyfill and styles to match v4
  • apply outline globally
  • check and customize focus for each component
• check for contrasts and improvements as suggested by @Aniort:
  1. size is 2px everywhere, with an offset to ensure spacing;
  2. contrasts should be good since we're using a single color scheme.
• ensure visibility by using box-shadow alongside outline to have two colors displayed: done for buttons, inspired by a comment in Bootstrap "Text and Non-text Contrast" issue — #29422
• document (mostly get docs back from v4-dev)

Anything else?

Questions

• Should form inputs use transitionned outlines too? For now they're stuck with border-color; and FWIW, .form-checks currently use both, which feels quite wrong… Edit: impossible, since :focus-visible specification understanding by browsers are that inputs should always show outlines. It won't sepearate :focus and :focus-visible, that's why they're still using border-color only.