Initially, the chat_mention db table was created to support notifications. So when creating
a `chat_mention` record we were always creating a related `notification` record. So did the
ChatMention fabricator.
Now we want to use the chat_mention db table in other scenarios. So we started decoupling
mentions from notification in 75b81b68.
This removes fabrication of Notifications from the ChatMention fabricator. We need to be able
to fabricate a ChatMention without a Notification.
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.
