Add support for real matrix notification using matrix-js-sdk

[PierreBesson]

The current notification system offers only webhooks which are good for Slack but to post notification to a Matrix server, you need to have webhook translation tool such as matrix-hookshot. Moreover those are less secure than sending message an authenticated bot account using the Matrix protocol.
This can be implemented by using the Matrix-js-sdk, see: https://matrix.org/docs/guides/usage-of-the-matrix-js-sdk

[lovelaced]

perhaps webhooks would be a better fit for a pagerduty system which may be better for this usecase.