Fix an issue where the "viewer" is not passed to Bulk Edit controls properly

Summary:
See PHI1442. If you have a bulk-editable datasource field with a composite datasource, it can currently fatal on the bulk edit workflow because the viewer is not passed correctly.

The error looks something like this:

Argument 1 passed to PhabricatorDatasourceEngine::setViewer() must be an instance of PhabricatorUser, null given, called in /Users/epriestley/dev/core/lib/phabricator/src/applications/typeahead/datasource/PhabricatorTypeaheadCompositeDatasource.php on line 231

Test Plan: Configured a Maniphest custom field with a composite datasource, then tried a bulk edit. Things worked cleanly instead of fataling.

Differential Revision: https://secure.phabricator.com/D20841