In ApplicationTransactionEditor, determine new objects with "getID()" instead of "getPHID()"

Summary:
See https://discourse.phabricator-community.org/t/unable-to-reload-object-that-hasnt-been-loaded/2677.

When editing "Config" objects, they currently get a PHID set outside of the TransactionEditor. They probably should not, but fixing that is likely an involved change.

This causes us to incorrectly fail to detect $is_new correctly and try to reload() and object with no ID.

To work around this, test for new objects with getID() instead of getPHID().

Test Plan: Edited any config value with the web UI.

Reviewers: amckinley

Differential Revision: https://secure.phabricator.com/D20482