Create a get_utilization() method on the RIR model

[jeremystretch]

Proposed Changes

Create a get_utilization() method on the RIR model to return the total utilization of all assigned aggregates, similar to what exists on the Prefix model.

Justification

RIR utilization is currently calculated by manipulating the queryset used in the list view. This is both inefficient and inflexible, because no convenient method exists for determining the utilization of a particular RIR. Moving this logic into a method is cleaner and allows us to remove the alter_queryset() method on ObjectListView.