Pass all non-default values as hidden field in namespace form on Special:Watchlist

Except 'namespace', 'invert' and 'associated' which are displayed in the form.
This allows to forward all parameters that were modified using query string, e.g.
the new 'extended' parameter added in I115544005a621ee01a0ed0970a13099af92f42e7
that would be lost otherwise when using the form.

Change-Id: Idb0d3b1d6b4827d35ac9b09eb4d8a5ae630a0e3a