Add builds minutes / hour limit

In https://gitlab.com/gitlab-org/gitlab-ce/issues/51401#note_200596653 we discussed about some limits to stop users from creating too many CI pipelines.

Problem

Understanding whether a customer (namespace) is abusing Gitlab.com resources is very hard. We tried looking into pipeline_size_limit (max number of jobs in a pipeline) and pipeline_activity_limit (how many pipelines are currently active). While these 2 limits could work as safe-guard for severe resource abuse they would be prone to false positive if set too low.

We also looked into jobs_activity_limit (number of jobs currently alive for the past hour).

Solution

It turns out that the primary CI limit could be calculated in builds minutes / hour.

This would count:

1. how long/often shared runners are kept busy for a specific namespace
2. how long/often Sidekiq workers are running in order to process jobs for a specific namespace
3. this will also cascade to other resource usage: DB, Redis, Workhorse, etc.

It could either be an iteration over jobs_activity_limit or an additional limit.

This could be tied up with the first iteration of CI/CD quotas