* Move L2VPN and L2VPNTermination models from ipam to vpn
* Move L2VPN resources from ipam to vpn
* Extend migration to update content types
* Misc cleanup
- Introduces a new `vpn` app with the following models:
- Tunnel
- TunnelTermination
- IKEProposal
- IKEPolicy
- IPSecProposal
- IPSecPolicy
- IPSecProfile