* Use WikiPage instead of Article

* Use WikiPage instead of Article
* Create the WikiPage object directly in execute() and pass it to both delete() and deleteFile()
* Reordered parameters of delete() and deleteFile() to be consistent and take both on the WikiPage and User objects (instead of relying on $wgUser); no callers outside of this class
* Fixed deleteFile() to match the behaviour of FileDeleteForm with oldimage parameter
* Pass the User object to getPermissionsError()
* Factorised duplicated code
* Added missing error code to ApiBase (and also a new one)
* Added missing possible error codes (with a new one too)