This diff updates our copy to use the term "channel" when referring to a chat inside a community. This will be used in robotext and notifs.

Depends on D6528

Flow and careful reading through the code. threadNoun is only used in two places – once in getNameForThreadEntity in entity-text.js, and once in notifTextForSubthreadCreation. I made sure that the relevant robotexts that end up using threadNoun through ET were updated