wporg-p2 Skip to Content link should be close to body tag

[hearvox]

This is the first of several tickets re: a11y deficiencies in the wporg-p2 theme at Make. I'm working on the A11y Handbook:
​https://make.wordpress.org/accessibility/handbook/

The "Skip to content" link is currently placed below the Menu and the headers. This means to get to this skip-link via keyboard navigation you must tab thru all the header and sub-header links, the search form controls, and all the menu links, before you can finally skip to the content.

To be useful this "Skip to content" link should be moved higher in the document, as close as possible to the opening body tag, e.g., as done in Twenty Sixteen and in Underscores:
​https://github.com/WordPress/twentysixteen/blob/master/header.php#L24
​https://github.com/Automattic/_s/blob/master/header.php#L23

[SergeyBiryukov]

There's a newer patch on #1484.

[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.