[web] Introduce add friends modal

Summary:
This modal allows adding new friends. Only users that do not have a relationship with viewer or are blocked are displayed as potential friends.

Depends on D4311

Test Plan: Render the modal and check if sending new friend requests work correctly.

Reviewers: def-au1t, atul

Reviewed By: atul

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4312