Media modals: Better responsive views, especially the header.

* Only show the "dropdown" when there is a menu. Note that this uses a `:not()` selector, but it is supported in browsers that support media queries so it is safe to use here.
* Make the header shorter when on particularly narrow screens, which also tend to be shorter.
* Don't restrict the height of images in the attachment details modal, to take advantage of all available space.

fixes #29339.

Built from https://develop.svn.wordpress.org/trunk@29632


git-svn-id: http://core.svn.wordpress.org/trunk@29406 1a063a9b-81f0-0310-95a4-ce76da25c4cd