Coalesce implementations of Stripe payment forms

[agjohnson]

This makes all of our implementations of payment forms use common bases, which
are also extendable into RTD.com. This implements several changes to how the
forms are used:

• Duplicate code was reduced by making views into model views
• Stripe functions were pushed into form processing to allow for better form
  validation error handling
• Common fields were moved onto a base class, ensuring the implementations all
  match
• The javascript interface was also cleaned up some, pushing more one-off
  pieces of code into the shared base
• Payment forms will report more errors, and will interpret errors from Stripe
  on the Stripe fields they are associated with
• A common field template for Knockout enabled forms is used on each form

The implementation for gold subscriptions changed slightly with this:

• Gold subscription form was turned into model form
• Views use model view create/update/delete views instead of manually
  duplicating that code
• Subscription deletion triggers a delete on stripe through a signal, to ensure
  that the stripe subscription always matches
• Redundant templates removed

The donation form implementation hasn't changed much, beside being updated to
use the base form and view classes.

Screenshots

Subscription Signup

Card rejection on subscription signup

Card rejection on CVV on subscription signup

Subscription details

Update subscription

Update subscription card showing

Confirm subscription deletion

Back to sign up, subscription cancelled

[ericholscher]

Is the split between Gold & Donate that one is recurring and the other is one time?

[ericholscher]

Seems weird we're setting this on the object directly at the top level in a util function?

Seems like it should go in the app config, or on the ready() function?

[agjohnson]

Yeah, split between donate and gold is that gold is a subscription and donate is a charge on Stripe. On subscriptions we need to track at least the customer id, on charge we don't need to track any object id -- though could be if we had any use for it on the RTD side.

[ericholscher]

Just seems weird to have 2 different places to give money, but I guess the "one time" vs "recurring" is different, but it seems odd to me as a user to have them in different places. I guess we could unify the UX while still having 2 diff forms.

[agjohnson]

Well, the donation page is only really a concern when we do an explicit push towards donations, so it's very unlikely that users would be confused by that. I left the link to the donation page on the subscription form, but not convinced we need that there.

[ericholscher]

Makes sense. We can just keep that as a backup, but Gold is the main thing.