Expand the "PhabricatorExternalAccount" table for new registration

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