Allow to test push notifications

[nickvergessen]

Example output:

$ occ notification:test-push admin 
Trying to push to 2 devices

Language is set to en
Private user key size: 1704
Public user key size: 451

Device token:9
Device public key size: 451
Data to encrypt is: {"nid":61,"app":"admin_notifications","subject":"Testing push notifications","type":"admin_notifications","id":"5e95bfdb"}
Signed encrypted push subject

Device token:11
Skipping talk device for non-talk notification

$ occ notification:test-push admin --talk
Trying to push to 2 devices

Language is set to en
Private user key size: 1704
Public user key size: 451

Device token:9
Skipping other device for talk notification because a talk app is available

Device token:11
Device public key size: 451
Data to encrypt is: {"nid":62,"app":"admin_notification_talk","subject":"Testing push notifications","type":"admin_notifications","id":"5e95bfdd"}
Signed encrypted push subject

Should help to find the problems e.g. with #619

[rullzer]

Didn't test but seems sane