Modernize "mailable" typeahead datasources

Summary: Ref T4420. Modernize the mailing list datasource, then build a composite "mailable" datasource.

Test Plan:
- Edited "subscribers" field in Differential revision edit.
- Edited "subscribers" field in Differential search.
- Edited "add subscribers" field in differential revision view.
- Edited "add ccs" field in Diffusion commit view.
- Edited "add emails to CC" in a Herald rule.
- Edited "add ccs" in maniphest bulk editor.
- Edited "add ccs" in maniphest task detail view.
- Edited "CC" on maniphest edit view.
- Edited "subscribers" on maniphest task earch view.
- Edited "CC" on pholio mock edit.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4420

Differential Revision: https://secure.phabricator.com/D9886

src/applications/diffusion/controller/DiffusionCommitController.php

@@ -771,6 +771,8 @@ final class DiffusionCommitController extends DiffusionController {
 
     require_celerity_resource('phabricator-transaction-view-css');
 
+    $mailable_source = new PhabricatorMetaMTAMailableDatasource();
+
     Javelin::initBehavior(
       'differential-add-reviewers-and-ccs',
       array(
@@ -783,9 +785,9 @@ final class DiffusionCommitController extends DiffusionController {
           ),
           'add-ccs-tokenizer' => array(
             'actions' => array('add_ccs' => 1),
-            'src' => '/typeahead/common/mailable/',
+            'src' => $mailable_source->getDatasourceURI(),
             'row' => 'add-ccs',
-            'placeholder' => pht('Type a user or mailing list...'),
+            'placeholder' => $mailable_source->getPlaceholderText(),
           ),
         ),
         'select' => 'audit-action',