PP-11212 Improve go-live and test account journeys

[stephencdaly]

Replace the 3 buttons on the services page to create live and test gateway accounts for Stripe/Non-Stripe with just 2 buttons:

• Go live
• Add test account

When "Go live" is clicked, determine the PSP from the go-live stage for the service.

If the service hasn't completed a go-live request, show an error.

When "Add a test account" is clicked, show a new page with radio buttons to select the PSP for the test account (Stripe/Worldpay/Sandbox)

On the page that asks for details about the service/gateway account before creating the gateway account in Pay:

• remove the radio button for Live/Not live, as we already know the answer now
• remove the radio buttons for selecting the PSP, as we already know the answer now
• prefill the description input, but still allow this to be edited
• hide the "Stripe credentials" section if we are not adding a Stripe account.