DEV: Make composer-uppy-upload mixin more extensible (PR #14138)

This mixin needs to be shared between the composer and composer-like user interfaces. This commit makes it so the events and the underlying data model is configurable by the component extending the ComposerUploadUppy mixin.

Also removes two MessageBus unsubscribe calls which were unnecessary.


Should that top level component unsubscribe from this.messageBus.unsubscribe("/uploads/composer"); when the component is destroyed?

I don’t see in code moment when we start listening to that queue, so probably it was some unused code but I would like to confirm with you

Yeah I removed that messageBus unsubscribe because I couldn’t see the channel being used anywhere, there is nothing subscribing to it.