[web] Introduce AddMemberGroup component

Summary:
Introduce component displaying group of users on the list in AddMembersModal - e.g. "Users in parent thread:", or "Not in parent thread"

Test Plan: Component can be tested after introducing "AddMembersModal"

Reviewers: palys-swm, atul, benschac

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3740