Add DoorkeeperExternalObject

Summary:
Ref T2852. This table holds data about external objects and allows us to write edges to them.

Objects are identified with an `<applicationType, applicationDomain, objectType, objectID>` tuple. For example, Asana tasks will be, e.g., `<asana, asana.com, asana:task, 93829279873>` or similar.

Test Plan: Ran storage upgrade.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6271