Implement user subscription credit budget

[ivelin]

Is your feature request related to a problem? Please describe.
Currently the auth0 user metadata tells us weather a user has an active subscription, but it does not help us determine how many notifications have been sent on behalf of the user. We need to limit exposure to potential bugs or hacks that result in unlimited spam.

Describe the solution you'd like
Add a "subscriptionPeriodCreditBudget" attribute to user profile that puts a limit on the number of premium services used during a monthly period. For example at the beginning of a month, the value can be reset to 100 and each email sent out on behalf of a user reduces the number by 1 (credit). At the beginning of the next period the number resets. In the future we can add other premium services such as SMS for 10 credits and so on.