Policy - filter app engines where the user can't see the application from panel editing

Summary: Fixes T7118. This does the basic "filter the list" thing, though it ends up being a little manual since I guess this hasn't come up before? There is also potential weird behavior if the user was using an app and lost access to it - they will have nothing selected on edit - but I think this is actually correct behavior in this circumstance.

Test Plan:
used a user who couldn't get access to the "quick create" apps and noted that the dropdown list on dashboard panel create was missing the expected engines

ran `arc unit --everything` to verify abstract method implemented everywhere

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7118

Differential Revision: https://secure.phabricator.com/D11687

src/applications/files/query/PhabricatorFileSearchEngine.php

@@ -7,7 +7,7 @@ final class PhabricatorFileSearchEngine
     return pht('Files');
   }
 
-  protected function getApplicationClassName() {
+  public function getApplicationClassName() {
     return 'PhabricatorFilesApplication';
   }