[web] Introduce Sidebar component

Summary:
Introduce component displaying single Sidebar item in the list in Sidebars Modal on web.

Test Plan: The component will be used in the following diffs introducing the modal.

Reviewers: tomek, atul, ashoat

Reviewed By: tomek, ashoat

Subscribers: ashoat, adrian, abosh

Differential Revision: https://phab.comm.dev/D4608