LDAP login/users better integration

[Marc Haase]

I'd really like Taiga to have better integration with LDAP.  We utilize Taiga in an enterprise and like to manage users using Active Directory.  I have hooked up and used the taiga-contrib-ldap-auth-ext on pip v0.3.4. (https://pypi.org/project/taiga-contrib-ldap-auth-ext/ which looks like it comes from https://github.com/Monogramm/taiga-contrib-ldap-auth-ext).

Some of the problems I'm facing with using that plugin is that it only really changes how authentication works and doesn't really have better ties to other parts of Taiga.  For example some of my pain points are:
* No good visibility/error reporting on why a user cannot login.  I've had to modify the taiga code to add some debug statements that help me troubleshoot these problems and I go look into the logs.  But it would be much nicer if it reported these types of errors to the user on the login page.  E.g. "user not found" or "password incorrect".

* User changes their profile username - this causes them to not be able to login, and when they login again it tries to create a new user with the old username but same email and Taiga has a restriction on the DB to have a unique email address causing login issues.  Ideally I would think if LDAP is being used these fields should not be editable.

* Signup page allowed people to create users (eventually I found out how to disable this in the config) that would eventually conflict with their LDAP account

[Marc Haase]

I guess another one might be that AD is case insensitive whereas Taiga is not.  So if a user logs in with SomeUsername today and someusername tomorrow it also ends up having a conflicting email record when they try to login

[Gerrit Bonn]

+1 the missing supported LDAP connector is currently looking to become the dealbreaker for us