[web] Introduce Members action in thread menu

Summary:
Activate displaying modal from thread actions menu.

Test Plan: The modal should appear after clicking on members in thread actions menu. It will be available only in some threads (like e.g. community threads, and not in personal)

Reviewers: palys-swm, benschac, atul

Reviewed By: palys-swm, benschac

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

Differential Revision: https://phabricator.ashoat.com/D3385