Email notifications for GitLab Flex usage

In previous versions of GitLab, you had no warning when your Flex usage approached a limit. GitLab now sends billing account managers an email notification when a product reaches 50%, 80%, or 100% of its monthly reservation, and when a capped product reaches its spend cap and is cut off for the rest of the billing period. Each threshold triggers at most one email per product per billing period, so you can act on overage early without getting repeat emails for the same threshold.