Allow changing document category as uploader

Currently a document's category cannot be changed unless you are a category admin or a super admin.

However, this restriction doesn't really make sense as anyone can download the document and create a new document in the new category. The current behaviour does not add restrictions to actions, it simply inconveniences the user, who may have accidentally uploaded to a wrong category.

While this issue is rather a small nitpick, I think it's a nice QOL change and it's not hard to implement, so I'm filing it as an issue!