Better checks for contributors when saving posts.

Merges [27964] and [27975] to the 3.8 branch.

props dd32, kovshenin, plocha.
see #27452.


git-svn-id: https://develop.svn.wordpress.org/branches/3.8@27976 602fd350-edb4-49c9-b593-d223f7449a82