wfMessage: use Message::params() to handle all the message parameters

wfMessage: use Message::params() to handle all the message parameters

The old code was similar to Message::params(), but Message::params()
was unable to handle "special" parameters and received an overhaul
in 7f2663f, yet wfMessage remained broken. To avoid duplication,
wfMessage shall call Message::params() to correctly handle these
parameters.

CategoryMembershipChange::getChangeMessageText and its caller has
been updated so as not to take advantage of this bug.

Bug: T153747
Change-Id: I6667acf7e71c9db07fefc9fbb741c160e15823ff