[lib][native][web] Add members of parent to sidebar when mentioned there

Summary:
This addresses ENG-3035, and does the core of what is needed for the whole project (ENG-2346): making it so when somebody is @-mentioned in a sidebar, then they are automatically added (as long as they're in the parent).

Depends on D6803

Test Plan:
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

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6804