Remove items which are not found in r2

This is definitely one to use `?w=1` with! (I replaced tabs with whitespace because Emacs wanted me to.)

Sometimes Pubflow alerts on items which don't exist, possibly because they have been launched and then quickly taken down. We now check to see if an item exists in r2. If it doesn't, we remove it from the watch list.

I still need to test this properly.