By doing this, we will:
* Have an open, but safe default People reach `@trust_level_1` pretty
quickly, but `@trust_level_0` is still excluded by default, to limit new
accounts joining and immediately spamming or otherwise abusing channels.
* Make it easier to change the default By keeping `@staff` in the
default, we make it easy for admins to remove `@trust_level_1` and
optionally add additional groups to their liking.
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.