Accessibility: Add an ARIA role `button` to links that behave like buttons when JavaScript is on.

Introduces a simple way to give a proper semantics of `button` to links that behave
like UI controls when JavaScript is on and behave like actual links when JavaScript
is off. First implementation on the Terms list table.

Patch prepared during WordCamp US 2015 Contributor Day.

Props mcapybara, garusky, takayukister.

See #26504.
Fixes #34867.
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git-svn-id: https://core.svn.wordpress.org/trunk@35911 1a063a9b-81f0-0310-95a4-ce76da25c4cd