Gallery Views: Avoid JS errors when image attachments lack metadata.

props gcorne.
fixes #27691.


git-svn-id: https://develop.svn.wordpress.org/trunk@28008 602fd350-edb4-49c9-b593-d223f7449a82
Formally deprecate wpmu_current_site().

props jeremyfelt.
fixes #27702.


git-svn-id: https://develop.svn.wordpress.org/trunk@28009 602fd350-edb4-49c9-b593-d223f7449a82
Improve inline documentation for functionality in wp-includes/plugin.php added in 3.9.

See #14994, #27700.


git-svn-id: https://develop.svn.wordpress.org/trunk@28010 602fd350-edb4-49c9-b593-d223f7449a82