Trim leading whitespace from AJAX responses.

This fixes the warning "$(html) HTML strings must start with '<' character" by jQuery Migrate when adding nav menu items, post custom fields or comment replies.

fixes #23681.
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git-svn-id: http://core.svn.wordpress.org/trunk@25466 1a063a9b-81f0-0310-95a4-ce76da25c4cd