Previously, restricted category chat channel was available for all groups - even `readonly`. From now on, only user who belong to group with `create_post` or `full` permissions can access that chat channel.
This is a followup of the previous refactor where we created two new
models to handle all the dedicated logic that was present in the
`ChatChannel` model.
For the sake of consistency, `DMChannel` has been renamed to
`DirectMessageChannel` and the previous `DirectMessageChannel` model is
now named `DirectMessage`. This should help reasoning about direct
messages.
