Remove redundant functionality from the admin UI

[jeremystretch]

Since its inception, NetBox has included a separate administrative interface that provides a sort of "behind the scenes" access to NetBox data. While this was a boon during nascent development, before all the different views which exist today had been fully fleshed out, today it is largely redundant. Further, actions taken via the admin UI are not subject to the same validation constraints as those processed via the normal front end.

I plan to remove all redundant functionality from the admin UI in v2.0. Redundant areas include most of the core apps: circuits, DCIM, IPAM, secrets, and tenancy. For example, the circuits list in the admin UI offers no additional features beyond what is available on the front end, so there's no reason to keep it around.

The remaining sections within the admin UI (namely authentication and extras) will remain intact until equivalent tools are implemented on the normal front end.