Updated mediawiki/extensions
Project: mediawiki/extensions/Mantle  4b698b44e08d240c8db21ce4d2ee4c9f75ec5d27

Hygiene: Cleanup Mantle documentation

Also:
* Remove dead code relating to default compiler
* Add singleton classes
* Wrap template compiler registration in closure
* Remove unnecessary jshint comments

Bug: 71250
Change-Id: Ib50391973555efb0dcc505e59e7bd555378bff0d