MQTT Client billing model

[knolleary]

This task is to cover the billing enablement of the MQTT Client feature #1350

This first iteration being delivered includes the ability to limit a Team Type to a given number of clients. From a billing perspective, this is the 'free' allocation that is included in the base team type cost.

Unlike our runtime billing model, which is based on realtime usage, the intended model is to allow users to pre-purchase additional 'blocks' of clients.

This task is to implement this billing model. It will require:

1. provide an API and UX flow for a user to purchase additional client blocks
2. track how many clients a team has purchased
3. update the 'create client' checks to ensure usage keeps within the combined free allowance and purchased allowance

[hardillb]

I ask the following question just to get it out of my head.

What happens if a user stop paying for extra clients?

Or do they need to reduce down to included number before they can?