implement LDAP authentication

[lhark]

Little by little the Rézo is trying to implement centralized LDAP authentication. For now it's used for the wifi and the gitlab. It would be great if the creation of an account on wifi.rez could allow you to connect to paiji.

I believe django does have a module handling this feature, however i don't know how this plays with existing users :/

Linked :
#34
#45

[LGD-Fr]

I only hope the über-centralized LDAP authenticating service does not use MD5 or SHA512 to hash passwords :-)

doc: https://docs.djangoproject.com/en/1.8/topics/auth/customizing/#authentication-backends
and: https://pythonhosted.org/django-auth-ldap/

It seems easy. But it increases automatically the chance paiji does not work :-)

[gpichot]

It can be just some configuration to add to the settings.py file. And I think there is still a model fallback when the LDAP is not available.
But it sill means a lot of migrations (for user password and cie).