Paid tier: hard budget cutoff

[ryanblock]

Is your feature request related to a problem? Please describe. Serverless products are known to leave a lot of folks apprehensive about the "denial of wallet attack" – paying for what you use can potentially result a big, unexpected bill at the end of the month. (In our experience, this hasn't been an issue, but it's a totally reasonable concern!)

Describe the solution you'd like We should have a budget cutoff for paid-tier services, where if the monthly bill is approaching the cutoff, warnings are fired; if it exceeds, we could begin to deactivate services.

/ht @andrewcowley!