Twenty Fifteen: clean up skip-link-focus-fix.js, see #29799

git-svn-id: https://develop.svn.wordpress.org/trunk@29897 602fd350-edb4-49c9-b593-d223f7449a82
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

git-svn-id: https://develop.svn.wordpress.org/trunk@29898 602fd350-edb4-49c9-b593-d223f7449a82