Themes: When only one theme is available, show just the details view.

Only one theme may be available when no other themes are allowed (multisite), no other themes are installed, or when the user is denied the switch_themes capability but still has edit_theme_options.

props matveb, markjaquith.
see #25962.

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git-svn-id: http://core.svn.wordpress.org/trunk@26542 1a063a9b-81f0-0310-95a4-ce76da25c4cd