wporg-p2 Skip to Content link should have visible text on focus

[hearvox]

The "Skip to content" link text, in the wporg-p2 theme at Make, should be visible when in focus state, i.e., when it's tabbed to using keyboard navigation (as, for instance, is the admin bar's "Skip to toolbar" text when you tab to it).

Right now you can tab to the "Skip to content" link but the text remains hidden, via a CSS clip. Removing the clip on focus will correct this a11y problem, e.g.:
.assistive-text.skip-link:focus { clip: auto; }

Note: another a11y-related ticket regarding this same link is: #1482

[SergeyBiryukov]

meta-1484.patch​ copies the .skip-link styles from Twenty Sixteen. Screenshot: meta-1484.png​.

The patch also moves the link closer to the start of the document, as per #1482.

[coffee2code]

In 2221:

W.org P2 child theme:: Move 'Skip to content' a11y link higher up on page and provide styling.

Adds .skip-link styles from Twenty Sixteen, giving the link visual styling and making it visible on focus.

Fixes #1482, #1484.
Props SergeyBiryukov.