This removes the modal container named-outlet/controller/template and replaces it with a component. Named outlets will be removed in Ember 4.x, so this change is part of that upgrade project.
Smaller changes include:
- update some of the computed values to be getters rather than calculated during `show()`.
- update tests which were previously depending on the modal class persisting after the modal was closed
Much of the logic in the service will be deprecated once we introduce component-based modals.
This work is split out from https://github.com/discourse/discourse/pull/21304
Previously merged in 80b77b2e and then reverted due to issues with the PM invite modal. This PR fixes the issue, and introduces a test which would have caught the issue.
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