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.
If we run this migration, everyone's ``last_topup_balance`` will be zero. An alternative approach would be to break this migration into three parts -- add ``last_topup_balance``, populate ``last_topup_balance`` from the ``alert_`` columns in a data migration, remove the ``alert_`` columns.

It's quite possible that it's okay for all the accounts to have zero ``last_topup_balance`` given that no one has billing notifications active yet and we'll need to load credits before we do that, but then we should be sure that we handle zero values for ``last_topup_balance`` well.