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.



git-svn-id: https://develop.svn.wordpress.org/trunk@26250 602fd350-edb4-49c9-b593-d223f7449a82