Allow CI limits to be defined at namespace-level

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This is extracted from a conversation here: #34634 (comment 234982006)

• Plan.default would work as default namespace limits on self-hosted
• Plan.<specific> is defined at namespace level for specific plans on Gitlab.com
• Admins should be able to override limits for specific namespaces that are prone to abuses (values must not be higher than Plan.default limits)

I'm a fan of moving limits out of namespaces and plans tables into dedicated namespace_limits and plan_limits having the same schema. Then, we could have namespace.actual_limits to be namespace.limits || namespace.plan_limits

This essentially allows fine tuning of limits at namespace-level. E.g. admins can drastically reduce limits (even temporary) for a namespace if this is being an abuser.