Summary: Ref T1536. Migrates the OAuthInfo table to ExternalAccount, and makes `PhabricatorUserOAuthInfo` a wrapper for an ExternalAccount.
Test Plan: Logged in with OAuth, registered with OAuth, linked/unlinked OAuth accounts, checked OAuth status screen, deleted an account with related OAuth.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T1536
Differential Revision: https://secure.phabricator.com/D6172
