Firefox Accounts login flow for developer pages (bug 1021295)

Turning on the `firefox-accounts` switch should make the login page and header login link go through the FxA login flow.
I wonder if we should have `settings.FXA_TOKEN_URL` and `settings.FXA_VERIFY_URL` based on the `FX_OAUTH_URL` instead of concatenating these here. If there's ever a v2 we can just update the settings in one place.