The OpenComposer mixin comes from a time before we had a composer service. As well as being a general cleanup/refactor, this commit aims to removes interlinking between composer APIs and the discovery-related controllers which are being removed as part of #22622.
In summary, this commit:
- Removes OpenComposer mixin
- Adds and updates composer service APIs to support everything that `openComposer` did
- Updates consumers to call the composer service directly, instead of relying on the mixin (either directly, or via a route-action which bubbled up to some parent)
- Deprecates composer-related methods on `DiscourseRoute` and on the application route
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