Currently when a category is deleted, if it has an associated chat
channel, the latter won’t be deleted automatically.
The fix is quite simple as we were simply missing a `dependent:
:destroy` option on the existing relation.
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.
Roman Rizzi
Loïc Guitaut