Notify user on some emergency situations

Some alerts, things that should get better notifications...

We should make it possible to subscribe to certain notification types

* Routers that are in our minimum working set that aren't responding
* Astronauts who have left the hab perimeter and aren't sending telemetry for ~15 minutes
* Astronauts who have sent one of the need assistance messages
* Astronauts who have sent a custom message (for now any custom message)
* Drone that appears when its team isn't visible
* Backup telemetry like APRS that reveals positions
* Aircraft below 1000ft
* Drone within 1km of hab

Lower priority alerts that we should only push if people can configure what alerts them.

* Astronaut leaves hab perimeter
* Astronaut leaves hab outside normal hours
* Repeater reports its location changes

We can send email and/or use pushover to notify on these situations.

Database changes needed
* Mark a relay station as essential
* Allow a user to subscribe to phone notifications
* Allow a user to subscribe to email notifications
* Allow a user to add an email to their account

No files, tests, or entry points are named. Start by locating the existing alert, user-account, database, email, and Pushover notification components, then map the listed emergency and configurable alert types to the required subscription and account fields. Done means the supported alert conditions can be configured per user and delivered by the selected channels.

Written by the indexing model from the issue text.