Allow packages to be added as revision reviewers via the UI

Summary:
Ref T10939. This lets you add packages as reviewers manually.

"Project Reviewers" now lists both projects and packages. I have renamed this to "Coalition Reviewers" but that's probably horrible and confusing. I'm not sure "Group Reviewers" is much better.

Test Plan:
  - Added a package as a reviewer manually.
  - Joined it, got authority over it.
  - Saw the review on my dashboard.
  - Accepted the revision, got authority extended to the package review.

{F1311652}

{F1311653}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10939

Differential Revision: https://secure.phabricator.com/D15914

src/applications/differential/customfield/DifferentialProjectReviewersField.php

@@ -8,7 +8,7 @@ final class DifferentialProjectReviewersField
   }
 
   public function getFieldName() {
-    return pht('Project Reviewers');
+    return pht('Coalition Reviewers');
   }
 
   public function getFieldDescription() {

src/applications/differential/customfield/DifferentialReviewersField.php

@@ -71,7 +71,7 @@ final class DifferentialReviewersField
     return id(new AphrontFormTokenizerControl())
       ->setUser($this->getViewer())
       ->setName($this->getFieldKey())
-      ->setDatasource(new PhabricatorProjectOrUserDatasource())
+      ->setDatasource(new DiffusionAuditorDatasource())
       ->setValue($phids)
       ->setError($this->getFieldError())
       ->setLabel($this->getFieldName());