DEV: switch to using subscription gem to avoid the need to rely on Subscription Client plugin

[merefield]

• IMPROVE: moves CW to rely on referenced gem instead of Subscription Client, so it can run entirely independently.
• IMPROVE: rationalises and consolidates subscription logic somewhat:
  • COMPATIBILITY: moves FE subscriptions management to a Service (in part because Mixins are deprecated)
  • IMPROVE: moves subscription serialisation to a dedicated controller on the BE.
• COMPATIBILITY: converts several related Components to Glimmer (and colocates template files as per standard).
• alters the way subscription client is mocked up in tests to accommodate the fact that overriding the class does not work the same way now that we've added the gem
• adds FE tests for Authorization and Subscription status.
• COMPATIBILITY: fixes deprecation of registerUnbound with Wizard Char Counter
• FIX: non working FE tests because of refactor of Core Composer has changed CSS selectors.
• IMPROVE: moves Pavilion icon to standard SVG registration system (from ad hoc template addition).

[merefield]

@angusmcleod, these front end tests are not failing locally on two types of browser, any ideas?

Otherwise this should be hot to trot.

[angusmcleod]

Not sure.

@jumagura Any idea why these tests are failing?

[merefield]

@jumagura please ignore, I found the problem, it's a breaking change from core due to updates to Composer and Modal selectors. This is currently also breaking Production tests, but I've fixed it here.

@angusmcleod this is now ready for review and merge.

[angusmcleod]

@merefield Two minor changes and a comment for future reference (no need to convert to .gjs in this PR).

[merefield]

both requested changes implemented @angusmcleod ...

[angusmcleod]

@merefield Looks like we've got a failing check here.

[merefield]

resolved @angusmcleod