Allow Admin to set default favorites for users

Migrating this request from GitHub to this new home for feature requests.

I want an admin control that can pre-set favourites for users, so that when they first join an RC instance, a set of important channels can be mode more obvious to them.

Users should be able to overide the default favourites if they choose to.

In my use case, I have many users who are quite unsophisticated with chat tech, and will benefit from me being able to make important content more obvious to them. Anyone who knows enough about the platform to modify their favourites settings probably does not need my help to find important channels and content.