Rename PhabricatorApplication subclasses

Summary: Ref T5655. Some discussion in D9839. Generally speaking, `Phabricator{$name}Application` is clearer than `PhabricatorApplication{$name}`.

Test Plan:
# Pinned and uninstalled some applications.
# Applied patch and performed migrations.
# Verified that the pinned applications were still pinned and that the uninstalled applications were still uninstalled.
# Performed a sanity check on the database contents.

Reviewers: btrahan, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: hach-que, epriestley, Korvin

Maniphest Tasks: T5655

Differential Revision: https://secure.phabricator.com/D9982

src/applications/repository/query/PhabricatorRepositorySearchEngine.php

@@ -8,7 +8,7 @@ final class PhabricatorRepositorySearchEngine
   }
 
   public function getApplicationClassName() {
-    return 'PhabricatorApplicationDiffusion';
+    return 'PhabricatorDiffusionApplication';
   }
 
   public function buildSavedQueryFromRequest(AphrontRequest $request) {