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/diffusion/controller/DiffusionCommitController.php

@@ -854,7 +854,8 @@ final class DiffusionCommitController extends DiffusionController {
     require_celerity_resource('phabricator-object-selector-css');
     require_celerity_resource('javelin-behavior-phabricator-object-selector');
 
-    if (PhabricatorEnv::getEnvConfig('maniphest.enabled')) {
+    $maniphest = 'PhabricatorApplicationManiphest';
+    if (PhabricatorApplication::isClassInstalled($maniphest)) {
       $action = id(new PhabricatorActionView())
         ->setName('Edit Maniphest Tasks')
         ->setIcon('attach')