Significantly simplify get_core_checksums(), as the caching and chunking was causing too much grief.

Make sure we only do our pre-flight is_writable check when the file exists.

see #18201. see #22704.

Built from https://develop.svn.wordpress.org/trunk@25801


git-svn-id: http://core.svn.wordpress.org/trunk@25713 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fourteen: better logic for featured image HTML output, and add fallback message for focusable anchor elements, for accessibility. Fixes #25325.
Built from https://develop.svn.wordpress.org/trunk@25802


git-svn-id: http://core.svn.wordpress.org/trunk@25714 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fourteen: link hover improvements, props sabreuse. See #25054.
Built from https://develop.svn.wordpress.org/trunk@25803


git-svn-id: http://core.svn.wordpress.org/trunk@25715 1a063a9b-81f0-0310-95a4-ce76da25c4cd