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.
Bikeshed: We seem to prefer generator / list comprehensions over ``map`` usually (I think because a lot of people find them more readable). So,
```
sorted(Decimal(str(lvl)) for lvl in app_settings.LOW_CREDIT_NOTIFICATION_LEVELS)
```
Not really necessary to change -- just pointing it out in case that's useful.