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.
Instead of passing `user` to `guardian.can_chat?`, we
can just use the inner `@user` that is part of the guardian
instance already to determine whether that user can chat,
since this is how it works for all other usages of guardian
even within chat.
Currently it’s not possible to delete a category if an associated chat
channel is present even if there are no messages in this channel.
This can lead to annoying situations for our users.
This patch addresses the issue by checking if the channel is empty
instead of just checking if there is a 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.
