[web] Introduce SidebarsModal

Summary:
Introducing new SidebarsModal component with two tabs - with all sidebars and those displayed in chat list (that we are member of)

Test Plan: The modal is not used yet, next diff replaces an old one with this component in menu and after clicking "see more sidebars".

Reviewers: tomek, atul, ashoat

Reviewed By: tomek, ashoat

Subscribers: ashoat, adrian, abosh

Differential Revision: https://phab.comm.dev/D4610