Support for 32-bit ASNs

[daknob]

Currently adding a 32-bit ASN throws an error message in the "Add Site" page:

Ensure this value is less than or equal to 2147483647.

However you are using a signed integer in this check. The BGP ASN can be a 32-bit unsigned integer, which means it can accept values from 0 to 4294967296. This error can render some ASNs unmanageable. Although RIRs have not allocated an ASN this large yet, the 32-bit ASN Private Range (4200000000 - 4294967294) is not supported.

[jeremystretch]

The asn field is currently defined as a PositiveIntegerField which as @daknob points out behaves as a signed integer. We might need a custom field to support unsigned 32-bit integers.