Display utilization of container prefixes based on child prefixes rather than IPs

[jeremystretch]

Issue type: Feature request

This idea was raised on the NetBox mailing list. Currently, prefix utilization is displayed as the percentage of consumed child IPs within it, regardless of the prefix's role. This doesn't make sense for container prefixes, which should have their utilization calculated in the same manner as aggregates: sum the space utilized by child prefixes and display it as a percentage relative to the parent prefix's size.

For example, a container prefix of 192.168.0.0/24 holding prefixes 192.168.0.0/26 and 192.168.0.64/26 should be considered 50% utilized, regardless of how many IP addresses have been created within it.