[web] Introduce block users modal

Summary:
This modal allows blocking users. Only users that are not blocked by the viewer can be blocked.

Depends on D4312

Test Plan: Render the modal and check if blocking users works correctly.

Reviewers: def-au1t, atul

Reviewed By: atul

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4313