[web] Introduce searching users in AddMembersModal

Summary:
Add logic responsible for finding users, that can be added (or not) to the thread based on search query. Used getPotentialMemberItems lib function, that is used in native app.

Test Plan: The search can be tested after introducing button activating AddMembersModal - search results should be exactly the same as in mobile app for the same query.

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/D3743