Summary:
Ref T1536. This is the schema code for `PhabricatorExternalAccount` which was previously in D4647. I'm splitting it out so I can put it earlier in the sequence and because it's simple and standalone.
Expands `PhabricatorExternalAccount` to have everything we need for the rest of registration.
Test Plan: Implemented the remainder of new registration on top of this.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T1536
Differential Revision: https://secure.phabricator.com/D6169
