push notifications for users (requires a user app install)

[ITJamie]

What problem would you like to solve? Please describe:
some of our users have very bad cell phone signal at their homes.
being able to send push notifications to users via a mobile app (or 3rd party push service eg pushover) would be great

Describe the solution you'd like:
a Mobile app with push notification support

Describe alternatives you've considered:
slack notifications with an @ username field filled out might also work? but requires being able to document the oncall persons slack username

Additional context:
similar to pagerduty's ability to send push notifications to users with the pagerduty app installed.

[mastercactapus]

This will likely come as part of #2639 which is currently in-progress.

However, for now, users can add SLACK_DM as an additional contact method from their profile page (escalation policies only support Slack channels). This will send them a DM over slack in addition to sms/voice/any other contact methods they have configured.