PP-13093: Changes to reflect the new test account system

[NathanD-GDS]

Context

When organisations create a service, we now ask them what type of organisation they are. Their answer will determine the type of test account we give them (Stripe or sandbox)

Changes proposed in this pull request

• Test your integration
  • Reword it so that the ‘default’ is for Stripe services to have a Stripe test account, Worldpay services to have sandbox test account
  • Reflect new process for requesting a Stripe test account
• Quick start
  • Mention that you need to add a new service (this should be there already but isn’t)
• Go live
  • Change wording of ‘tell us your PSP’ to something like ‘confirm your PSP’ because organisations have already chosen their PSP during service creation