add lang support for drafts (#826 & #1107)

Add lang support to drafts, this can't be done in a very clean way as you can see in the code so either I rescan the file or take the `content, metadata, settings, source_path, context` from the article instance and copy it. But `Article.__init__()` does some manipulations on `metadata` for example so I decided to go for the former. It's not ideal but it works and should not affect the rendering so much as the number of drafts should never be very high in practice (let's hope).

Fix #826
Fix #1107