Change behavior of Special:Watchlist when user's watchlist is empty

Previously it would display nothing but a message saying that "You
have no items on your watchlist.", not even any information about what
we are looking at or a link to edit watchlist's contents.

Now the full view is displayed, with the old message placed instead of
"N pages on your watchlist, not counting talk pages." at the top.

Change-Id: Ice9be8bb0686c3c20712d814f08a5171380bf45d
Merge "Change behavior of Special:Watchlist when user's watchlist is empty"