Created PhabricatorExternalAccount Class and Sql patch to create an external_account table.

Summary: Created PhabricatorExternalAccount class with only data members. Will discuss with you regarding the necessary functions to be implemented in this class. Sql Patch to create a new table for external_accounts. Will I have to write unit tests the new storage object? Sending you this diff so that you can comment on this to further improve :).

Test Plan: {F40977}

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, AnhNhan

Maniphest Tasks: T1536, T1205

Differential Revision: https://secure.phabricator.com/D5724