[web] introduce member list sidebar

Summary:
This diff introduces the member list sidebar component. Subsequent diffs will handle adding this component to the chat panel

Linear task: https://linear.app/comm/issue/ENG-5975/introduce-new-member-list-section

Depends on D10921

Test Plan:
Please see the demo video below (the duplicate users here are just for testing purposes + not a part of this diff)

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10922