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.
I think we should probably sort after converting to ``Decimal``, becase e.g.
```
>>> sorted(Decimal(str(i)) for i in x)
[Decimal('-1.5'), Decimal('2.0'), Decimal('10.0')]
>>> [Decimal(str(i)) for i in sorted(x)]
[Decimal('-1.5'), Decimal('10.0'), Decimal('2.0')]
```
(I know we're expecting the config to be floats, but it won't complain about strings).