PLAT-541 Add support for replace query param via global and URL param

Add support for ?replace=true query param and `$wgVignetteReplaceThumbnails` global. These settings, when set to something truthy, will force the thumbnailer to replace the cached (disk) version of the thumbnails that are generated.


The vignette side is https://github.com/Wikia/vignette/pull/33. The corresponding ticket is here https://wikia-inc.atlassian.net/browse/PLATFORM-541.

/cc @nmonterroso @garthwebb