Add support for matrix notifications

[gdt]

See #449 for background; this is basically the same feature request but for matrix instead of XMPP.

This issue is a feature request for notifications via matrix. This is related to #370, which is more than notifications and might not be matrix.

The implementation should support

* configured MXID and password, and the usual server discovery
* a MXID to which alert messages should be transmitted
* when offline, queuing of messages locally and retrying the server connection
* optional heartbeat messages in some matrix-compatible way, perhaps via presence, so one can get alerts on connection failure
* of course, there should be no use of FCM (some parts of the matrix world seem to think it's ok to use FCM, hence mentioning this)