[web] Introduce SubchannelsModal

Summary:
Introduce modal with subchannels list.
Currently, it doesn't display message previews from threads that user is not member of - the next diff will introduce fetching proper data from server

FIgma:

Test Plan: Component will be accessible from thread menu after the diff introducing "Subchannels" action into thread menu.

Reviewers: palys-swm, benschac, atul

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3600