Use sprites for (nearly) all application icons

Summary: Sprites for everyone.

Test Plan: Loaded `/applications/`.

Reviewers: btrahan, chad, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1569

Differential Revision: https://secure.phabricator.com/D3280

src/applications/base/PhabricatorApplication.php

@@ -55,7 +55,11 @@ abstract class PhabricatorApplication {
   }
 
   public function getIconURI() {
-    return PhabricatorUser::getDefaultProfileImageURI();
+    return null;
+  }
+
+  public function getAutospriteName() {
+    return 'default';
   }
 
   public function shouldAppearInLaunchView() {