Before we relied on the combined data being present in ipsets.json
when generating a new config, but ipsets.json is only updated through
the form at /ipsets. So submitting any other form after changing
NetBox definitions might crash when trying to find an entry from
networks.json in ipsets.json.
Now we introduce a helper functon to always read both files and
combine the prefixes fron networks.json with ipsets.json. This way it
is not necessary to save a new ipsets.json before other changes.
Also don’t crash when enumerating networks for each VPN group.
