Use a tokenizer, not a gigantic poorly-ordered "<select />", to choose repositories in Owners

Summary: Depends on D19190. Fixes T12590. Ref T13099. Replaces the barely-usable, gigantic, poorly ordered "<select />" control with a tokenizer. Attempts to fix various minor issues.

Test Plan:
  - Edited paths: include/exclude paths, from different repositories, different actual paths.
  - Used "Add New Path" to add rows, got repository selector prepopulated with last value.
  - Used "remove".
  - Used validation typeahead, got reasonable behaviors?

The error behavior if you delete the repository for a path is a little sketchy still, but roughly okay.

Maniphest Tasks: T13099, T12590

Differential Revision: https://secure.phabricator.com/D19191

src/applications/diffusion/controller/DiffusionPathValidateController.php

@@ -45,19 +45,6 @@ final class DiffusionPathValidateController extends DiffusionController {
       'valid' => (bool)$valid,
     );
 
-    if (!$valid) {
-      $branch = $drequest->getBranch();
-      if ($branch) {
-        $message = pht('Not found in %s', $branch);
-      } else {
-        $message = pht('Not found at %s', 'HEAD');
-      }
-    } else {
-      $message = pht('OK');
-    }
-
-    $output['message'] = $message;
-
     return id(new AphrontAjaxResponse())->setContent($output);
   }
 }