Add the ability to drag and drop files directly onto the editor.

The file will then begin to upload and the media manager will open.

props kovshenin.
see #19845.

Built from https://develop.svn.wordpress.org/trunk@27343


git-svn-id: http://core.svn.wordpress.org/trunk@27195 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Strip backslashes, not just forward slashes, from untrailingslashit().

trailingslashit() will now remove any forward or backslashes from the end of a string before appending a forward slash.

props knutsp, willmot.
fixes #22267.

Built from https://develop.svn.wordpress.org/trunk@27344


git-svn-id: http://core.svn.wordpress.org/trunk@27196 1a063a9b-81f0-0310-95a4-ce76da25c4cd