💄🔧：lift the navbar links off the ink

[DerekNonGeneric]

Bootstrap's dark navbar draws its links at white 55%. Against our ink that
measures 6.04:1 — it clears AA, but only just, and at that size it reads washed
rather than quiet.

alpha   effective          vs ink
0.55    rgb(153,152,150)    6.04     <- was
0.78    rgb(205,204,204)   10.93     <- now
1.00    rgb(255,255,255)   17.46     <- hover and current page

78% is nearly double the contrast and still sits back from the page it is
pointing at.

The part that came with it

The current page was marked by being white among greys. Once the greys are
lifted to 78% that distinction nearly vanishes, and it was a weak marker to
begin with: colour alone, no second cue, and nothing announced to a screen
reader.

So the current link now takes a rule under it in $primary, and the
aria-current="page" that should always have gone alongside the active
class. Marked twice, and one of the two is not visual.

Scope

Both rules are .navbar-dark, which is the fixed top bar and nothing else. The
mobile sidebar is a plain .navbar on $secondary, so its links were never
part of this, and it already marks the current page with a border down its
left.

aria-current is added in both navs, since the sidebar deserves it equally.

Checks

nps test passes, all 17 verifiers. vnu clean on /, /about/ and
/news/. Confirmed in the built output that active and aria-current land
on Home from / and About from /about/.

How it came up

Reviewing the home page in #1833. It is site-wide, so it is here on its own.

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