[web, native] Introduce and use ThreadIsChannel function

Summary:
Introduce helper function as a follow-up of D3191, that simplifies logic for conditionally displaying Subchannels menu item.

Depends on D3191

Test Plan: Tested web and mobile app. Subchannels menu seems to work in the same way as before.

Reviewers: palys-swm, ashoat

Reviewed By: palys-swm, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3261