Importing Twenty Fifteen, first pass at our new default theme for 2015, set for 4.1.

It's good for posts, it's good for pages, it might be good for you. Development will occur in trunk. Have at it.

Props matt, iandstewart, iamtakashi, obenland, cainm, kristastevens, karmatosed, chellycat, lancewillett, kwight, davidakennedy. See #29799


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git-svn-id: http://core.svn.wordpress.org/trunk@29648 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: JSHint fixes.
 * Define global variables set by wp_localize_script().
 * Remove currently unused $ variable.
 * Use dot notation when appropriate.
 * Update Gruntfile.js to ignore the html5 shiv.
see #29799.

Built from https://develop.svn.wordpress.org/trunk@29893


git-svn-id: http://core.svn.wordpress.org/trunk@29649 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Set Twenty Fifteen as the new default theme. see #29799.

Built from https://develop.svn.wordpress.org/trunk@29895


git-svn-id: http://core.svn.wordpress.org/trunk@29650 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Support 'EXISTS' and 'NOT EXISTS' in `WP_Tax_Query`.

These new values for the 'operator' parameter make it possible to filter items
that have no term from a given taxonomy, or any term from a given taxonomy.

Includes unit tests.

Fixes #29181.
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git-svn-id: http://core.svn.wordpress.org/trunk@29651 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: clean up skip-link-focus-fix.js, see #29799
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git-svn-id: http://core.svn.wordpress.org/trunk@29652 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Admin menu:
- Fix pinning after resizing the window.
- Merge the two DOM ready callbacks in common.js
- Fix the submenus position adjustment on focus.
See #29806
Built from https://develop.svn.wordpress.org/trunk@29898


git-svn-id: http://core.svn.wordpress.org/trunk@29653 1a063a9b-81f0-0310-95a4-ce76da25c4cd