[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