When a user saves his or her preferences, we save all settings, even if the user
didn't change from the default.
However, if the admin changes the global default, those users' preferences stay
as the old default.
It would be nice if the user doesn't really care about the setting, we didn't
save it. This is probably not possible with HTML forms, so as a first
approximation we could only save preferences that differ from the defaults.
The downside is that if the user really loves the old default, they'll have to
go change it when the global default changes. This seems like a preferable
situation to the current behaviour.