Add support for past_due subscriptions

[knolleary]

Fixes #1878

As I originally thought this was going to collide with the changes in #1889 I chose to based this work on the fix-coupons branch. In hindsight, I think the changes are somewhat more self-contained so don't strictly need to be based on that branch... but they both need to go in regardless.

Description

This adds track of 'past_due' stripe subscriptions. This is the state they go into as soon as a payment has failed.

With this PR, we update our internal state to 'past_due', but we still treat that as 'active' as far as all the billing logic goes. This ensures no change in functionality at this point in time. We may choose to restrict behaviour in the future - but not as part of this PR.

This then allows us to add a banner notifying the user if there is an over due payment:

This also includes a related fix where, when a subscription is canceled and we suspend all projects, we weren't resetting the billing state. This prevented them being re-added to new subscription once setup.

Related Issue(s)

#1878

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on flowforge/helm to update ConfigMap Template
  • Issue/PR raised on flowforge/CloudProject to update values for Staging/Production

Labels

• Backport needed? -> add the backport label
• Includes a DB migration? -> add the area:migration label

[Pezmc]

Overall looks good! But the test failures are related, but this does need test coverage added!

[knolleary]

Yup - looking at the tests now.

[knolleary]

Have updated the tests around handling the Stripe callback - was bad luck the tests were using past_due as an example unsupported state... but I should have spotted it regardless.

Also added a test that verifies a past_due subscription still reports as active - so all existing behaviours that rely on the active state are not affected.

@Pezmc good for another review