Local header deletion on web upload

If you delete a header locally and then do -W, --web upload, the article is currently not removed from web.

Instead, we simply make its content become empty, and mark it as unlisted.

The reason for this is that the article may have metadata created by other users such as OurBigBook Web discussions, which we don't want to delete remove.

In order to actually remove the header you should follow the procedure from Section "OurBigBook Web page renaming", which instead first moves all discussions over to a new article before deleting.