[web] Display list of pending users to add in AddMembersModal

Summary:
Introduce list of pending users to add below search bar in AddMembersModal.
The diff also excludes pending users from main list in Modal, so they don't appear twice.

Test Plan: Open "Members Modal", try to add users from list, pending users appears below search bar.

Reviewers: palys-swm, atul, benschac, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3876