Migrate the OAuthInfo table to the ExternalAccount table

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