backends/ldap: add lookup by username, make password change work with Django models
backends/ldap: add lookup by username, make password change work with Django models

- When using username lookup, the linked dns are updated
- LDAPUser is now a proxy class of the user model
- User model are always pickled and stored inside the session user id
- We do not try anymore to fix collision, the realm is made for this.
	The username template must be written to be unique and username lookup
	should be preferred. DN lookup can be used as the first method if DN
	are garanteed to never change.

pu239ppy/authentic2,pu239ppy/authentic2,pu239ppy/authentic2,pu239ppy/authentic2