Improvements to the modal media uploader on mobile in landscape orientation.

* Merge the 782 and 500 width queries into a single 640px query
* At this breakpoint, remove the drop shadow and margin from the uploader modal
* For only landscape views, tabs are shown in a single line.
* Add padding to tab links to account for untappable 20px area at top of landscape viewport in iOS
* Add overflow: visible to .embed-link-settings at responsive sizes to prevent unnecessary framed scrolling

See #25977, props joen.


Built from https://develop.svn.wordpress.org/trunk@26250


git-svn-id: http://core.svn.wordpress.org/trunk@26156 1a063a9b-81f0-0310-95a4-ce76da25c4cd