Unbreak OAuth Registration

Summary:
@vrana patched an important external-CSRF-leaking hole recently (D1558), but
since we are sloppy in building this form it got caught in the crossfire.

We set action to something like "http://this.server.com/oauth/derp/", but that
triggers CSRF protection by removing CSRF tokens from the form. This makes OAuth
login not work.

Instead, use the local path only so we generate a CSRF token.

Test Plan: Registered locally via oauth.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran, epriestley, demo

Maniphest Tasks: T853

Differential Revision: https://secure.phabricator.com/D1597