Move Session model to use the NetBox core ASN model

[christianpinger]

ported @nahun's commit to latest k01ek/netbox-bgp@develop
Credits: Nathan Wheeler nathan.wheeler@zoom.us
nahun@6b870b0

NetBox v3.1 added a native ASN model. The plugin should utilize that model instead of a separate model that is specific to BGP.

Fixes #82

[christianpinger]

I've added data migrations for ASN objects.

A native ASN object must be created for every ASN used in a BGPSession, before running this migration. Afterwards every Session will reference to the native ASN object.

[k01ek]

Thanks @christianharendt and @nahun for your work, in next release I provide tool for migration plugin ASN objects to native NetBox ASN objects. So plugin ASN model will be removed later, I think in next minor release.

[christianpinger]

Thanks @k01ek
I was also considering to create a native ASN object for each plugin ASN during migration, but the native ASN model has a mandatory field: RIR. I was unsure what to insert there as a safe default. Maybe the first RIR found or create a meta RIR "netbox-bgp"?

[k01ek]

@christianharendt yea, just create some default RIR. I think change RIR for ASN is easier than create all ASN numbers by hands.

[christianpinger]

I've updated the code to be compatible with the latest develop commits