Revert r41710, r41978, r42012, r42048 (integration of Poem extension to core plus...

Revert r41710, r41978, r42012, r42048 (integration of Poem extension to core plus misc. fixes). Per Wikitech-l, this isn't properly attributed to the original authors of the extension. Furthermore, "Poem" is entirely too narrow a name for the functionality this tag provides. If this is to be integrated, a better name should be chosen.