Update Asana feed publishing integration for "ExternalAccountIdentifier"

Summary: Depends on D21017. Ref T13493. Update the Asana integration so it reads the "ExternalAccountIdentifier" table instead of the old "accountID" field.

Test Plan: Linked an Asana account, used bin/feed republish to publish activity to Asana.

Maniphest Tasks: T13493

Differential Revision: https://secure.phabricator.com/D21018