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 don't think so. The old path used the `upgrade` flag to control the copy shown to upgrading users. The new path controls the copy shown to those upgrading by explicitly sending users to `verify_student_upgrade_and_verify`; the `upgrade` flag doesn't appear to be used.