WIP: Send a type="normal" notification on highlight when not joined
This is for persistent channels where the user has no client joined. In that case, there is no way of letting a user know that they have been mentioned, while to the IRC side, it appears that they are online and available (and in fact, they may be active in another channel).
To make this situation better, this introduces a notifcation message of type="normal" which includes the mention, the channel where it came from and the sender; this notification is only sent if the user has no clients in the channel.
- Send body notification/summary text
- Add tests
- Send HTML version with join URIs
- Test this in real-life