Twenty Twelve: Move skip link to be first focusable element

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/62967

This PR fixes an accessibility issue in the Twenty Twelve theme by moving the "Skip to content" link to be the first focusable element on the page. No changes to the link's functionality or styling.

[github-actions]

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[sabernhardt]

Suggested change

	<a class="assistive-text" href="#content"><?php _e( 'Skip to content', 'twentytwelve' ); ?></a>
	<div id="page" class="hfeed site">
	<header id="masthead" class="site-header">
	<div id="page" class="hfeed site">
	<a class="screen-reader-text skip-link" href="#content"><?php _e( 'Skip to content', 'twentytwelve' ); ?></a>
	<header id="masthead" class="site-header">

1. I prefer placing the link inside the "page" div (as Twenty Fifteen and some later bundled themes do). If someone uses a link to #page, then the skip link would be the first item.
2. If you also change the assistive-text class to screen-reader-text, Twenty Twelve already has styles for .site .screen-reader-text:focus. Then style.css would not need any editing.

[himanshupathak95]

Hey @sabernhardt Thanks for the suggestions!

I have also updated the changes and moved the skip links inside the page div for the Twenty Thirteen and Twenty Fourteen themes. Please let me know if the changes look good to you whenever you have time.

Thanks.