create multiple low credit notification levels

Currently there is only one notification level. Add a list in the config of all the notification percentage levels that should be checked, and send the relevant notifications when the levels are reached.
We can have `ENABLE_LOW_CREDIT_NOTIFICATION` in each of the tests, but `LOW_CREDIT_NOTIFICATION_LEVELS` is read when the class is created, so it has to be here. 

I did notice that these should be `GO_...`, so I'll change that so long.