[Feature Request] Change language of a document

Hi there,
After uploading a document, it's currently impossible to change the language that was set to it while uploading. Be it because of a simple mistake, the source not supporting it (you wouldn't actually go and set a source for every language, will you?) or because that language didn't exist in Docspell when you uploaded it (wink, wink), I think it's a necessary feature to save the hassle of deleting and reuploading a bunch of document :)

Thanks!