Make traces feature optional via traces_disabled setting

Make traces feature optional via traces_disabled setting

Adds an optional traces_disabled setting to allow alternative deployments to disable the traces/tracks feature entirely. When absent (the default) traces are enabled; set to true to disable.

When disabled, all trace routes are removed via a routing constraint so requests return 404. Navigation links to traces in the header and user profile are conditionally hidden. CanCan abilities for Trace respect the setting.