[web] Introduce ThreadMembersList component

Summary:
Create component with list of thread members grouped by first letter of username. The component will be displayed in modal.
The component disallows list scrolling if any menu from its children is open (that's why it keeps opened menus list).

Test Plan: The component can be tested after introducing members modal.

Reviewers: palys-swm, benschac, atul

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, benschac

Differential Revision: https://phabricator.ashoat.com/D3383