Media library: Unify headers between view modes.

Includes back compat for `views_upload` links. These look a little awkward, but do not appear to be commonly used. Also prevents the view mode links from being dropped on smaller screens so a user doesn't get "stuck" in the list view.

This is different from other list tables, but aligns well with other uploaded content management screens (themes, plugins).

props pento, helen.
fixes #28946.

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git-svn-id: http://core.svn.wordpress.org/trunk@29399 1a063a9b-81f0-0310-95a4-ce76da25c4cd