Restrict teammates to specific brands in multi-brand workspaces

In a multi-brand Enterprise workspace, allow admins to restrict which teammates can reply to conversations for a given brand. For example, some teammates should only be able to access and reply to Brand A conversations, while others can be granted access to Brand A and Brand B. Today, Assignments controls routing (who a conversation gets assigned to) but does not enforce a hard restriction on who is allowed to reply to a given brand's conversations. Teammates without brand access can still reply. Requested by a prospect running a white label, multi-brand support setup on behalf of their own customers, where different support staff should only be able to touch specific brands' conversations.