Redirect loop on `/tokenauth/tokens`

Steps to reproduce:

1. Start RelengAPI with default settings.
2. Open `/tokenauth/tokens` in your browser.
3. You'll be redirected to `/login_request?next=%2Ftokenauth%2Ftokens`.
4. Log in with some valid credentials.
5. Observe the redirect loop:

![screen shot 2014-12-16 at 5 31 31 pm](https://cloud.githubusercontent.com/assets/44974/5456064/791d4e0c-8549-11e4-92da-ebb51ec9c93b.png)