Add url param 'extended' to Special:Watchlist

This new url param allows overridding of the user option
'extendwatchlist' in the same way than, for example, 'enhanced' for
'usenewrc'.
This is helpful to provide a extended enhanced watchlist per url.
Without extendwatchlist a enhanced watchlist is not useful (no page
groups possible)

Also fixed the default for param namespace

Change-Id: I115544005a621ee01a0ed0970a13099af92f42e7