Add aria-current attribute to current item in WP Admin <nav> tabs

[audrasjb]

WordPress Admin screens that use tabs navigation should use aria-current attribute on <nav> elements.

For full details, see #47094

Milestoned in 5.2.1 since this regression have been introduced in 5.2 (see #43398)

Current implementation:

    <li>
        <a href="...">...</a>
    </li>

Recommended code:

    <li>
        <a href="..." aria-current="page">...</a>
    </li>

   a[aria-current] {
        /*  styles */
    }

[audrasjb]

That should be fixed in:

• Menu screen
• About.php screen – Edit: + Credits, Freedoms, Privacy :)
• Edit site screen (in multisites)

[audrasjb]

adds aria-current attribute to Admin nav tabs

[afercia]

In 47106.2.diff​:

• adjustments for the wp-admin/nav-menus.php page
• makes phpcs happy
• fixes some (pre-existing) missing translators comments to make phpcs even happier

[afercia]

In 45300:

Accessibility: Add aria-current to the links in the <nav> elements introduced in [44905].

Also fixes a few translators comments and coding standards.

Props audrasjb.
Fixes #47106.

[afercia]

Reopening for 5.2.1 consideration.

[desrosj]

In 45318:

Accessibility: Add aria-current to the links in the <nav> elements introduced in [44905].

Also fixes a few translators comments and coding standards.

Merges [45300] to the 5.2 branch.

Props audrasjb, afercia.
Fixes #47106.