Add trial user E2E Testing & ensure redirect correct after creating first application

[joepavitt]

Description

• Removes unnecessary loading = false just before the redirect, as that was triggering a re-mount of the InstanceForm and in turn redirecting to /billing
• Adds testing infrastructure for checking platform state with trial users.
  • Moves the stripe mocking into a central location for forge unit tests, and the new ee E2E tests.
  • Adds a new terry user who will be a user on trial for 5 days.
  • Adds tests to ensure message is shown when Terry logs in, that they can create an Application/Instance and that they're redirected to /billing when trying to create a second

Related Issue(s)

Closes #2285

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass

[joepavitt]

Currently blocked as can't work out how to configure a trial user in our front-end E2E tests