DAR-1394 | Special:Promote - changed from FakeLocalFile into UploadStash

https://wikia-inc.atlassian.net/browse/DAR-1394

UploadStash doesn't work with our external thumbnailer and MW default thumbnailer does not have cropping functionality which is used on Special:Promote, so we introduced WikiaUploadStash which overrides some core methods to use our thumbnailer. 

Special:Promote now uses WikiaUploadStash to upload temporary files.

@federico-lox - please take a look on those changes.