Deprecate 'maniphest.enabled' and 'phriction.enabled'

Summary:
Also join concepts of installed and enabled applications.
Also respect uninstalled Maniphest where disabled Maniphest was checked.

Test Plan:
Visited T1, D1.
Uninstalled Maniphest then visited T1, D1.
Disabled Maniphest then visited T1.
Visited /config/edit/maniphest.enabled/.

Reviewers: epriestley, Afaque_Hussain, edward

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5602

src/applications/differential/controller/DifferentialRevisionViewController.php

@@ -529,7 +529,8 @@ final class DifferentialRevisionViewController extends DifferentialController {
         'sigil' => 'workflow',
       );
 
-      if (PhabricatorEnv::getEnvConfig('maniphest.enabled')) {
+      $maniphest = 'PhabricatorApplicationManiphest';
+      if (PhabricatorApplication::isClassInstalled($maniphest)) {
         $links[] = array(
           'icon'  => 'attach',
           'name'  => pht('Edit Maniphest Tasks'),