[web] Add buttons that open new modals

Summary:
Add label and callback props to user list modal so that new modals can be opened. Set these props in block / friend list to appropriate modals.

Depends on D4313

Test Plan:
Open friend list and check if it has a correct button that opens a modal where new friends can be added.
Open block list and check if it has a correct button that opens a modal where users can be blocked.

Reviewers: def-au1t, atul

Reviewed By: atul

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4314