GraphQL fixes: handling for BigIntegerField (ASNField) and ContentType

[glennmatthews]

Fixes: #428

For fields of type BigIntegerField (currently, AFAIK, only used as our subclass ASNField), use a custom scalar type that allows arbitrarily large integer values. This pattern will probably fail for values over 2^53ish since that is the limit supported by JavaScript, but as ASNField only allows numbers up to 2^32-1, that's a non-issue at this time.

Verified in GraphiQL that large ASN values are accepted as query parameters and are also permissible in the returned data:

Added regression test coverage as well.

[glennmatthews]

Second commit fixes another GraphQL issue: because the built-in ContentType model naturally isn't decorated with @extras_features("graphql") it's not covered by our automatic generation of DjangoObjectType definitions. As a result, fields that referenced ContentType were being omitted from GraphQL. Adding an explicit type definition fixes this:

[glennmatthews]

One more fix: enable the IPAddress.assigned_object field. This is a GenericForeignKey field, which graphene-django does not know how to automatically handle. Since it can point to either an Interface or a VMInterface, this field needs to be defined as a Union type, which we previously had no examples of in our code base. It does work quite nicely now: