Plugin details modal:

* Scroll all content, rather than the dual-pane scrolling awkwardness.
* Better align the header image and title overlay with the display on WordPress.org, at least at full-width.
* Move the close button outside of the modal itself, rather than overlay on top of colors we can't predict.

props stephdau, tellyworth, helen. fixes #27440.

Built from https://develop.svn.wordpress.org/trunk@29474


git-svn-id: http://core.svn.wordpress.org/trunk@29252 1a063a9b-81f0-0310-95a4-ce76da25c4cd