[feature] Send alerts for found systems

[AceMgy]

Mostly writing this here so I don't forget.

Basically the idea is to have something in the settings where a user (or an entire user group if an admin is doing this) could set up alerts if a particular system is connected in a map you have view access too.

Spitballing here, I'd just ForeignKey the system to a new table that goes something like:

class SystemAlerts(models.Model):
     users = models.ManyToManyField(<whatever the user model is>)
     groups = models.ManyToManyField(<whatever the group model is>)
     alert_added = models.DateTimeField(auto_now=True)
     alert_target = <whatever the slack hook for a channel is>

Then just have some logic in the function that adds systems that checks this table.

Edit: and of course I only see issue #160 after I submit this.