Created a preference pane for DarkConsole, instead of link

Summary: Just removed the link and created a new field under preferences. Now the setting is under Display Preferences.

Test Plan: Enablied/Disabled dark console to see if it works.

Reviewers: epriestley

Reviewed By: epriestley

CC: irinav, aran, Korvin

Maniphest Tasks: T2344

Differential Revision: https://secure.phabricator.com/D4549

Conflicts:

src/view/page/PhabricatorStandardPageView.php