Wikicite 2017: ‘extends’-attribute for ref-tag

This presentation was used as part of a lightning talk at Wikicite 2017 promoting the plan by the WMDE community tech team to implement an extends attribute for the ref tag in wikitext.

The aim of the presentation was to prompt questions and discussion.

This relates to phabricator ticket https://phabricator.wikimedia.org/T151301