Don't show the DarkConsole preference if is false in the config

Summary: D4549 added a preference option for DarkConsole, but we should only show it if darkconsole.enabled is true. In most installs DarkConsole is not enabled, so now we check before showing it in the display settings so it's not confusing for users.

Test Plan: tested on localhost, when we change the darkconsole.enabled value in ENV the option is not displayed.

Reviewers: epriestley

Reviewed By: epriestley

CC: irinav, aran, Korvin

Differential Revision: https://secure.phabricator.com/D5069