Rename Ci::Minutes::Quota to Ci::Minutes::Usage

Problem

With the introduction of a new Ci::Minutes::Limit class we now have Ci::Minutes::Quota class that represents the SSoT for any CI minutes model.

Since the name "quota" can be confused with a "limit" this is now ambiguous. The Limit class represents anything to do with limits (monthly/purchased) while the Quota class is responsible also for the usage.

Solution

We could rename Ci::Minutes::Quota to Ci::Minutes::Usage where in turn the Usage class delegates to Limit for anything to do with limits (as it happens today).

Then we can still access the same object from namespace: namespace.ci_minutes_usage and namespace.ci_minutes_usage.limit