This allows us to define a hook that will get called on the client after a message is successfully created. See more context in ENG-3034.

Note that for each MessageType that needs this, a callsite will have to be manually added. That's because we handle message creation differently for each composable type.

Depends on D6802

I tested this stack with the following steps on both web and native:

1. Make sure users get auto-added to a sidebar when @-mentioned
2. Test mentions typeahead and make sure it includes all members of parent
3. If a user is @-mentioned in a text message that triggers the creation of a sidebar, then make sure they are included in the sidebar creation message
4. If a user is @-mentioned in a text message that triggers the creation of a sidebar, then make sure their name is bolded from the very start of when the message is visible on-screen