[web] Add option to select pending users

Summary:
Introduce a set to which we add users after clicking their button. Filter out the selected users so that they can be added only once.

Depends on D4307

Test Plan: Render the component and add some users - every user should disappear from the list after being added.

Reviewers: def-au1t, atul

Reviewed By: atul

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4308