Combine payment and intro step

Based on UX feedback from @andywaldrop.  When we're including payment steps, we want to combine the "intro" (requirements) step with the "make payment" step.

To do this, I've factored out the "requirements" HTML into its own view and underscore template.  This view gets used by both `IntroStep` and `MakePaymentStep`.  I've also updated the Python code to exclude the intro step if there's a "make payment" step.

Reviewers: @stephensanchez @rlucioni