Don't unregister User admin

[bufke]

https://github.com/crgwbr/asymmetric_jwt_auth/blob/master/asymmetric_jwt_auth/admin.py#L17

I think it's a bit intrusive and would cause problems if my application also unregistered User admin.

I think it would be better to have just a separate section for public keys. They would rarely be added anyway. That is also how django rest framework's auth tokens work.