Internalize storage access for PhabricatorUserOAuthInfo

Summary:
Ref T1536. Move all access to the underlying storage to inside the class. My plan is:

• Migrate the table to ExternalAccount.
• Nuke the table.
• Make this class read from and write to ExternalAccount instead.

We can't get rid of OAuthInfo completely because Facebook still depends on it for now, via registration hooks.

Test Plan: Logged in and registered with OAuth.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6171