Plan the following changes, ask questions to clarify before implementing

Re-factor the notification system:
- use available libraries for pushover, matrix, email and sms notifications.
- notifications have a title/subject:  alert_type (recover/warning/critical), a body (info from threshold check) and a link to the host plugin metrix page
- define a list of notification channels for each user
- notifications are dispatched to users that are listed as managers for the host



1 - correct
2 - for now channels are defined globaly 
3 - matrix-nio)sounds good, homeserver URL, access token, room ID per channel?
4 - use the REST api provided by https://voip.ms/api/v1/rest.php
5 - The page does not exist yet, point at the host tab in the /plugins
6 - per-channel minimum severity is a good idea, go fo it
7 - yes

1 - use base_url, there might not have been any incoming requests yet
2 - use same asyncio loop for matrix-nio
3 - for now, just silently do nothing