The mailer in charge of sending chat summary emails applies a filter to ensure only members of groups listed in the `chat allowed groups` setting receive them. However, when you set it to `everyone`, nobody will be notified because
we treat this group differently and don't create `GroupUser` records for every user on the site.
This commit changes the mailer to skip the filter when the `everyone` ID is in the list.
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.
