IPAM inheritance fixes

[glennmatthews]

Fixes: #N/A

This PR ports fixes from NetBox for two closely related bugs:

• Set VRF for available IPs to same VRF as parent prefix netbox-community/netbox#5703, in which adding a child IPAddress to a Prefix would not default the new IPAddress form selections to inherit the parent Prefix's VRF and Tenant values
• Inherit VRF for child prefix from parent prefix netbox-community/netbox#6012, in which adding a child Prefix to a Prefix would not default the new Prefix form selections to inherit the parent Prefix's VRF and Tenant values

These fixes are direct ports of the original fixes by @nkeulen and @jeremystretch respectively.

In both cases the fix boils down to a correction of the table column templates to correctly reference object (the Prefix which is being viewed, and which will serve as the parent object of the newly created IPAddress or Prefix) instead of nonexistent variables parent or prefix.