bug 43220: use mediawiki:listings-template to override formatting

Modify mw:extension:listings to look for mediawiki:listings-template
as the name of a template to be used to format individual listings
instead of using the built-in defaults.

Change-Id: I90dced07924d0b87cb7737fdf969203fae8810ca
Merge "bug 43220: use mediawiki:listings-template to override formatting"