Modernize "projects" typeahead datasource

Summary: Ref T4420. Update "projects" source.

Test Plan:
- Edited projects on a Differential revision.
- Edited projects on a commit.
- Edited projects on a repository.
- Edited projects in feed search.
- Edited projects in a Herald rule field.
- Edited projects in a Herald rule action.
- Edited projects in Maniphest batch editor.
- Edited projects on Maniphest task.
- Edited projects in "Associate Projects..." action in Maniphest.
- Edited projects on Maniphest search in "all projects", "any project" and "not projects" fields.
- Edited projects on a Paste.
- Edited projects on a Pholio mock.
- Edited projects on a custom policy rule.
- Edited projects on a Ponder question.
- Edited projects on a Diffusion search query.
- Edited projects on a global search query.
- Edited projects on a slowvote.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4420

Differential Revision: https://secure.phabricator.com/D9884

src/applications/paste/controller/PhabricatorPasteEditController.php

@@ -186,7 +186,7 @@ final class PhabricatorPasteEditController extends PhabricatorPasteController {
         ->setLabel(pht('Projects'))
         ->setName('projects')
         ->setValue($project_handles)
-        ->setDatasource('/typeahead/common/projects/'));
+        ->setDatasource(new PhabricatorProjectDatasource()));
 
     $form
       ->appendChild(