Hook up new payment and verification flow

Replaces #6253. This modifies the "verify now" link on the dashboard, the track selection page, and the "upgrade" link on the dashboard to point to the new split flow. This doesn't modify the postpay callback called after the user returns from CyberSource. There are also many tests which expect the old verification flow; I'm holding off on updating them until we know what we're doing about the A/B test.

@wedaly, could you take a look at this when you're able?
I think we'll have to include the value of the session variable in the context passed to this template. We can then check that value along with the value of the feature flag to determine which link to use.