Adding Handle Suport

Summary: Added PHID_TYPE_XUSR to loadObjectsofType() of PhabricatorObjectHandle data. Was trying to solve the setActor() issue. Created a getPhabricator method in PhabricatorExternalAccount. When I try to set authorPHID using setAuthorPHID() in PhabricatorExternalAccount it's saying bad setter call, if I don't, then it's saying authorPHID can't be null. Sending you diff for comments on this.

Test Plan:
{F42523}

{F42525}

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, AnhNhan

Differential Revision: https://secure.phabricator.com/D5839