[web] Introduce add user item

Summary:
This component will be used on friend and block add lists.

Test Plan: Render the component on a modal and check if it looks correctly and reacts to user click action

Reviewers: def-au1t, atul, benschac

Reviewed By: atul

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4266