[web] introduce member list sidebar header

Summary:
This diff introduces the member list sidebar header component.

For context here is what the figma designs for this look like

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

Depends on D10899

Test Plan:
Please see the demo video below:

Reviewers: atul, inka, ashoat

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10900