* Introduce RelatedObjectCountField
* Introduce get_annotations_for_serializer() and enable dynamic annotations
* Add RelatedObjectCountFields to serializers; remove static annotations from querysets
* Remove annotations cleanup logic from BriefModeMixin
* Annotate type for RelatedObjectCountField
* Remove redundant field on TagSerializer
* Add missing reverse relationship for power feeds to rack
* Refactor RelatedObjectCountField to take a single relationship name
* 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
