Add a payment request screen for Orders with the "Pending" status (OCC-188)

[sarahelsaig]

Followup to #117

Is your feature request related to a problem? Please describe

Currently we can create new Orders from the content item editor, but there is no way to have them paid from withing OCC. If you create a custom order it has to be paid outside of the site and then the order manually updated.

Describe the solution you'd like

Add a new action to the PaymentController (e.g. ~/checkout/request) that displays the payment UI (e.g. the card widget from Stripe) so the payment can be fulfilled by the customer. Also add a button to the Order display view if the status is pending.

Describe alternatives you've considered

It could be possible to display the payment controls right in the Order display if needed, but I think that's too much complexity.
I also considered automatically redirecting the user to the payment request screen if the order is pending, but that may be confusing. Also it can be achieved using stock OC workflows if that's a desired behavior so there is no need to force it out of the box.

Jira issue