[web] Fixed visual bug with disappearing buttons

Summary:
Fixing: https://linear.app/comm/issue/ENG-2057/userlist-buttons-are-cutoff-when-there-are-a-lot-of-users
With many users and short browser window, buttons were disappearing.
Fixed by using flexbox.

Test Plan: Video above.

Reviewers: tomek, kamil, michal, atul

Reviewed By: tomek

Subscribers: ashoat, atul, michal, kamil, tomek

Differential Revision: https://phab.comm.dev/D5724