Use :focus-visible to define styles on focus

Styles on keyboard focus are essential for keyboard navigation; without
them, keyboard users wouldn't see which element they're currently
interacting with. That's why we use an `outline` on elements having the
current keyboard focus.

However, this is sometimes annoying for mouse/touchscreen users, since
clicking/touching an element also gives it keyboard focus.

When clicking on a button performing some kind of action through
JavaScript, keeping the outline on the button after clicking it is
distracting.

Even after clicking a link, for some users having the outline present
while they wait for the next page to load is annoying.

That's why modern browsers (at the time of writing, 74%) implement the
`:focus-visible` pseudoclass, which selects elements which have received
focus using the keyboard, but not elements which have been
clicked/tapped on. We can use it to provide focus styles for keyboard
users without getting in the way of mouse/touchscreen users.

Usually we wouldn't use a feature which isn't supported in more than 96%
of the browsers out there. However, in this case we've got a solid
fallback: we just use the `:focus` pseudoclass. Since the `@support` CSS
condition doesn't accept pseudoclasses as parameters, we're disabling
`:focus` styles only on browsers supporting the `:focus-visible`
pseudoclass using the `:focus:not(:focus-visible)` selector, which will
be ignored by browsers without support for `:focus-visible`.

Since now users receive less feedback when clicking/touching a link or a
button, we're adding styles for the `:active` pseudoclass. This way
users will know which item they're clicking/tapping on. I'm not sure the
outline is a good option for this case, though; I think for touchscreen
users a better solution would be to apply the styles we apply on hover.
We might change it in the future.

Note grouping styles together like this would *not* work:

```
&:focus,
&:focus-visible {
  // Styles here
}
```

Browsers not supporting the `:focus-visible` pseudoclass would ignore
this statement completely, meaning they wouldn't apply the styles on
`:focus` either.

app/assets/stylesheets/layout.scss

@@ -116,6 +116,19 @@ button,
   &:focus {
     outline: $outline-focus;
   }
+
+  &:focus-visible {
+    outline: $outline-focus;
+  }
+
+  &:focus:not(:focus-visible) {
+    outline: none;
+  }
+
+  &:active,
+  &:focus:active {
+    outline: $outline-focus;
+  }
 }
 
 .button {