[web] Introduce Subchannels action in thread menu

Summary:
Introduce action in thread menu, that launches subchannels modal

Test Plan: In thread actions menu subchannels action should be available for thread that contain some subchannels

Reviewers: palys-swm, benschac, atul

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, atul, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3602