Support for announcement-only channels

It would be nice to have something like an announcement-only channel where
only the channel admin are allowed to post new messages and everyone else
has read-only access. I would picture this being used for things like
company-wide announcements where you want to make sure it can only be used
for that purpose and avoid having those messages lost amongst unrelated
conversations that belong elsewhere.

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*Derek Gottlieb*
Sangoma | Cloud Janitor | Engineering / Switchvox Cloud Dev