This is just a little clean-up in tests. In the past, when creating a `chat_mention`
record, we always created a related notification. Starting from fa543cda
notifications and chat_mentions are fully decoupled from each other. So if we're
testing just chat mentions there is no need to fabricate notifications for them.
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.
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.
