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.
The text in the email is:

`Please note that your account {{account}} has reached {{threshold_percent}}% of the total available credits.`

Maybe we should change that to `{{threshold_percent}}% used of the total available credits` or `{{threshold_percent}}% remaining of the total available credits` depending on which way we choose?