Provide a LDAP backend option for login

[Salamandar]

It would be nice to be able to login via a LDAP. A lot of orgs provide their LDAP. Obviously, we're talking about petition authors here, not signatories.

If enabled, LDAP shouldn't be mandatory. What I mean is that a lot of softwares use the ldap backend as "read-only" and also accept "local registration".
Also, if the LDAP is enabled, local registration could be disabled by the administrator, to ONLY use the ldap.

Maybe this is a good ldap backend for django : https://django-auth-ldap.readthedocs.io/en/latest/