Move L2VPN models to new vpn app

[jeremystretch]

Proposed Changes

FR #9816 (VPN tunnel modeling) has introduced a new vpn app. This issue proposes moving the L2VPN and L2VPN Termination models and all their related resources from the ipam app to the vpn app.

Justification

There was discussion at the time when L2VPN support was being added about whether it made sense to create a new app for it. Although we opted not to at the time, it would be prudent to move these now as the vpn app is a much more suitable home for these models.

Unfortunately, this will effect a breaking change in the REST API, however it should be easily manageable.

[abhi1693]

Will bookmarks remain unaffected?

[jeremystretch]

I expect related objects, content types, etc. to all be unaffected; that's the plan at least. We've never actually moved a model between apps before IIRC, but I don't think it will be too disruptive.