[web] Introduce Subchannels thread menu actions

Summary:
Added Subchannels list and create options. It appears depending on user permissions - in the same way as in native.
The onClick functionality will be added in a separate diff.

Test Plan: Tested if items appears if user has proper permissions in thread.

Reviewers: palys-swm, benschac, atul, ashoat

Reviewed By: palys-swm, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3191