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 
does this need a default? unless `$wgVignetteReplaceThumbnails` is set or the `$_GET` param is present and truthy, `$replaceThumbnail` is never set, which would probably give us the good ol' "unset variable" notice. Would still work, I think, but we should prevent this warning :)