Copy Gadget:ReferenceTooltips from enwiki

Specifically: https://en.wikipedia.org/w/index.php?title=MediaWiki:Gadget-ReferenceTooltips.css&oldid=1234748639 https://en.wikipedia.org/w/index.php?title=MediaWiki:Gadget-ReferenceTooltips.js&oldid=1315353040

The gadget is normally enabled by default but here we're importing it as optional.