Added site_url template variable in order to help generating pages that require a full absolute URL.

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extra : rebase_source : 7095107d738701524f68c1f9e0086d3fab199286
Added support for adding addional HTML attributes to the "linkify" template filter.

--HG--
extra : rebase_source : 639c86567c3ed4bb0ad090913b24669762fc3894