Child prefixes change order in the aggregate's list when Hide available is selected

[markkuleinio]

Environment

• Python version: 3.7.3
• NetBox version: 2.7.11

Steps to Reproduce

1. Create RIR for example "RFC1918"
2. Create Aggregate 10.0.0.0/8 with RIR RFC1918
3. Create VRF for example "Test VRF"
4. Create prefix 10.1.0.0/16 in "Test VRF"
5. Create prefix 10.2.0.0/16 with no VRF (= Global)
6. Go to Aggregates - 10.0.0.0/8
7. See that the list shows prefixes in IP-ordered: 10.1.0.0/16, 10.2.0.0/16 (as well as available prefixes)
8. Click Hide available

Expected Behavior

The active prefixes would still show in the same order: 10.1.0.0/16, 10.2.0.0/16, just the available prefixes hidden

Observed Behavior

The prefixes are shown in reverse order 10.2.0.0/16, 10.1.0.0/16 (i.e. Global prefix is shown first, VRF second)

[DanSheps]

I think this might be related to the same issue with IP Prefix hierarchy getting broken, FYI.

[jeremystretch]

Tagging as possibly related to #4617