Disable external entities in ID3.

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Media modal/grid: get the proper .attachment width in arrow navigation. Props avryl, see #27423.
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git-svn-id: http://core.svn.wordpress.org/trunk@29157 1a063a9b-81f0-0310-95a4-ce76da25c4cd
TinyMCE: add support for `class` attribute on the caption shortcode. Part props knutsp, kadamwhite, fixes #28951.
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git-svn-id: http://core.svn.wordpress.org/trunk@29158 1a063a9b-81f0-0310-95a4-ce76da25c4cd