Return false from metadata_exists() if the get_$type_metadata filter returns a false value.

props xknown.
fixes #22746.

Built from https://develop.svn.wordpress.org/trunk@27562


git-svn-id: http://core.svn.wordpress.org/trunk@27405 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Cap and theme support checks for the welcome panel.

props MikeHansenMe.
fixes #26276.

Built from https://develop.svn.wordpress.org/trunk@27563


git-svn-id: http://core.svn.wordpress.org/trunk@27406 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Comments dashboard widget: Use the proper class to avoid stretched "unapproved" red lines.

fixes #26910.

Built from https://develop.svn.wordpress.org/trunk@27564


git-svn-id: http://core.svn.wordpress.org/trunk@27407 1a063a9b-81f0-0310-95a4-ce76da25c4cd