[web] Introduce Subchannel component

Summary:
Introduce component for displaying single subchannel with preview in thread subchannels modal

Figma:

Test Plan: The component is not used yet. The component can be tested when introducing subchannels modal.

Reviewers: palys-swm, benschac, atul

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3599