Port oref0-pushover functionality to NS

[scottleibrand]

Currently oref0-pushover is used for carbsReq notifications (and supports insulinReq notifications as well). Doing so from the rig itself has some drawbacks:

• Each rig alerts independently, so with multiple rigs you get too many alerts for the same thing.
• There is no way to use any other alert mechanism other than Pushover.

In order to support more flexible alerting, I think we should probably move the oref0-pushover functionality into NS, and use NS alerts to do other new things like alerting on pump error conditions like No Delivery etc.

[alimhassam]

One small disadvandage i see is that there seems to be a longer delay in NS notifications vs the direct carbsReq ones from openaps. I'm sure this could be worked around somehow, but just wanted to add a note about this.