Cleanup from code review; fix test suite

Addresses the code review comments on #6364, fixes the test suite, and cleans up some of the code that's no longer used.

Removing the "progress" steps makes this flow *much* simpler.  I think we could probably simplify this code a lot more, but at this point I'm taking a minimalist approach.

@andywaldrop I have a few more questions about the design comps.  In particular:

* What happens if someone ends up on the "make payment" page without selecting an amount on the previous page?  I think in that case, we'd want to show the payment option.

* The "already verified" case on the "make payment" page looks ridiculous, because no requirements are displayed.  How should we handle this case?

* There was some text that said "we will review over the next (x) hours".  As far as I know, we don't make promises anywhere else about how long verification will take, so I'm very curious what value we need to put there (if any).  I know we're waiting on final copy from marketing, and I'm worried about accidentally leaving this in, so for now I've removed that part of the sentence.

Reviewers (when you get back from vacation): @AlasdairSwan @stephensanchez 
I know this string hasn't been touched as part of this PR, but I noticed that elsewhere, you've changed instances of "register" back to "enroll" ([aside](https://i.imgflip.com/1bhk.jpg)). This sentence still refers to course enrollment as registration.