[web] Add cancel and confirm buttons
Summary:
The cancel button closes the modal. The confirm button adds either friends or blocks the users.
Test Plan:
Render the component and check if the buttons look ok.
Confirm button should be disabled while there are no users selected.
Cancel and confirm button should call appropriate functions.
Reviewers: def-au1t, atul
Reviewed By: atul
Subscribers: ashoat, Adrian, yayabosh
Differential Revision: https://phab.comm.dev/D4310