R1:bce4b7addfce

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…