Rename Watchlist request parameters for consistency with RC's ones

This is necessary for us to be later able to have them be inherited
(and use FormOptions). Old parameters (and thus old URIs) will continue
to work (but will never be used in newly generated links).

However, this breaks compatibility of the SpecialWatchlistQuery hook, as
the keys of the array it accepts as the last parameter have changed due to
the renaming. This could be worked around easily if we deem it worth
worrying about.

Change-Id: Ia6b2047006c86256ec22d7eda92a23be2058f1a3