Ensure invoice does not go above Usage Limit

[easyCZ]

We want to ensure that the number of credits on an invoice is at most the value of the spending limit. This is to provide users with a guarantee that they will not be charged for more than their spending limit.

To achieve this, we will need to do the following:

• When updating a spending limit, prevent setting it below any existing accrued usage. The spending limit can be increased, but it cannot be decreased below the current usage.
• When updating invoices, retrieve the spending limit for each Attribution ID and take the `min(usage, spending_limit)

See related (internal) conversation.

[laushinka]

• When updating a spending limit, prevent setting it below any existing accrued usage. The spending limit can be increased, but it cannot be decreased below the current usage.

Do we need a design for how we show this message to the user? cc @jldec @gtsiolis

[laushinka]

Unassigning and moving to next week's board based on this internal discussion.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.