If a user starts a sidebar by replying to a message with a new text message, and @-mentions somebody in the text message, then that person gets added to the sidebar when it is created.

However, we have a weird visual behavior today where the person's name doesn't appear bolded while the thread is being created. That's because the pending ThreadInfo that the MessageListContainer is using isn't being updated to include the @-mentions.

This diff adds some plumbing to make this possible.

Depends on D6809

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