[native] Allow only parent thread members to be added to a sidebar

Test Plan: Checked if user list in compose-thread and add-users is displayed correctly, checked if for sidebar clicking on user that is not in parent thread alert is displayed but for the rest of threads is the same as before

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D528