Provide a way for administrators to change Confluence's default time zone

Currently Confluence's default time zone can only be set by a system property, which effectively requires file system access and an application restart.

The ability for normal administrators to set this option is of particular importance to OnDemand. At the moment, if a customer wants the default time zone changed, a Support engineer has to book a maintenance window with the customer for the outage and organise a HOPS engineer to change the container's system time zone, which is not a trivial amount of work.

It would be good if a customer could just change the default time zone in the application's general configuration, as is the case with JIRA.