Allow apostrophes in email addresses when adding users via the Dashboard.

Email addresses entered in a number of interfaces were not being stripslashed
properly, with the result that the emails were not being recognized as valid.

Fixes #18039.
Built from https://develop.svn.wordpress.org/trunk@29966


git-svn-id: http://core.svn.wordpress.org/trunk@29713 1a063a9b-81f0-0310-95a4-ce76da25c4cd