Export same-origin check and `end_of_redirect_response` template.

Hello there,

I'm trying to add support to the OAuth redirect login flow to the alternative DDP client [asteroid](https://github.com/mondora/asteroid).

I'm encountering difficulties mainly for two reasons:

1. [only same-origin redirects are supported](https://github.com/meteor/meteor/blob/f4e100dc146d233521e2084c7457dd4e32c53c0f/packages/oauth/oauth_server.js#L376)
1. [`endOfRedirectResponseTemplate` is private](https://github.com/meteor/meteor/blob/f4e100dc146d233521e2084c7457dd4e32c53c0f/packages/oauth/oauth_server.js#L277)

The proposed changes solve those issues, as they allow me to overwrite those properties (using the package [mondora:asteroid](https://github.com/mondora/mondora-asteroid)).
Is the solution acceptable? Let me know if it needs any work.

Thanks
Paolo