Multiheader editing

Maybe we should only do this after: github.com/ourbigbook/ourbigbook/issues/248 to prevent data loss.

One possibility is to prevent deletion/renaming of headers. We could just check the new ID list agains the previous ID list.

This was possible previously on Web, but we forbade it for simplicity of implementation sake.

We can then think about how the UI would look like, there might be a "Edit article and descendants" button on toplevel only for example.