Add option to load-preferences to omit disabled accounts.

This should provide more optimization for the fmn backend.

Currently, it holds a cache of the preferences of every user, and for every
message that comes by, it evaluates the rules for every user.  This
introduces some unnecessary steps when, for instance, @nirik has his irc
message delivery disabled.  We still evaluate *all* of his irc rules
against *every* message
(including the thousands of secondary arch koji messages that no-one is
interested in being notified about).

By removing filters such as @nirik's disabled irc rules from the
message-by-message considerations, we should see some small performance
increase.