Revert "[fix] Informations du membre: Gestion du cas où un utilisateur n'est pas rattaché à un membre"

This reverts commit 1d930a99.

This commit inadvertently removed the change_password() function.

It's still not perfect, see comment in the code.

With this, administrative subscriptions and configuration backends are
clearly separated.  We use LDAP in the configuration backends, but they
can be implemented with anything else.  For some usages, no configuration
backend is even needed.

Genericity is hard to achieve with SQL.  The implementation here is
somewhat unsatisfying and error-prone: it uses implicit constraints which
are not enforced at the database level.  Fortunately, Django provides the
tools to enforce such constraints in most cases anyway.

This is still pretty raw, and subject to changes.  A first backend is
implemented directly (VPN), but in the future, a mixin class will handle
the details of the relation between configuration backend and
administrative subscription, since it works the same for every backend.

Signed-off-by: CapsLock <faimaison@legeox.net>

Signed-off-by: CapsLock <faimaison@legeox.net>

Signed-off-by: CapsLock <faimaison@legeox.net>

Signed-off-by: CapsLock <faimaison@legeox.net>

Signed-off-by: CapsLock <faimaison@legeox.net>

Signed-off-by: CapsLock <faimaison@legeox.net>

Signed-off-by: CapsLock <faimaison@legeox.net>

Signed-off-by: CapsLock <faimaison@legeox.net>