[lib] Fix sendnig create_sidebar to peers

Summary:
issue: ENG-9134
For sidebars we should also inform the members of the parent about thier creation

Test Plan: Tested that now when a sidebar is created in a thick thread, other members of the thick thread are informed

Reviewers: tomek, kamil

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13259