Move LDAP authentication into a standalone authentication backend

[jeremystretch]

Proposed Changes

With the introduction of support for custom authentication backends (#2328) in NetBox v2.8, it no longer makes sense to maintain LDAP authentication as part of NetBox's core.

Justification

Offloading this functionality removes the burden of maintenance from the core maintainers, encourages more community participation in the development of the LDAP backend itself, and should serve as a strong reference implementation for custom authentication backends. It will also help guide the development of remote authentication support in NetBox.

[bluikko]

I'll repeat here that it would be nice if the LDAP module could also be used as an authorization module when external authentication is used. I don't know how doable/complex that would be.

After authentication is completed externally the user's group memberships are read from LDAP for permissions.

[jeremystretch]

@bluikko That is beyond the scope of this issue.

[Haeki]

Since its possible to using an LDAP backend as custom authentication backend thats seems like a good idea.
A Problem could be if someone wants to use both ldap and another custom authentication backend. A possible solution would be to allow the use of more than one custom backend.

[Dimaqa]

I believe you need to add REMOTE_AUTH_CONFIG now, because there is no way to specify LDAP config other than in settings.py. Or am i missing something?

[sly-net]

I'll repeat here that it would be nice if the LDAP module could also be used as an authorization module when external authentication is used. I don't know how doable/complex that would be.

After authentication is completed externally the user's group memberships are read from LDAP for permissions.

@bluikko I've made a proof of concept of this. Check it out here: https://github.com/sly-net/netbox-remote-authn-ldap-authz