Update `wp_kses_bad_protocol()` to recognize `:` on uri attributes,

`wp_kses_bad_protocol()` makes sure to validate that uri attributes don’t contain invalid/or not allowed protocols. While this works fine in most cases, there’s a risk that by using the colon html5 named entity, one is able to bypass this function.

Props: xknown, nickdaugherty, peterwilsoncc.

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git-svn-id: https://core.svn.wordpress.org/trunk@46695 1a063a9b-81f0-0310-95a4-ce76da25c4cd