Support for Active Directory multidomain forest

In AD forest samAccountName (or uid) may not be unique in the
entire forest and userPrincipalName contains "@" symbol
disallowed in Matrix User Identifiers.

This commit reflects changes in ldap_auth_provider that adds
mxid generation logic for Active Directory.

Signed-off-by: Yuri Konotopov <ykonotopov@gnome.org>

ma1sd.example.yaml

@@ -165,6 +165,8 @@ threepid:
 # ldap:
 #   enabled: true
 #   lookup: true # hash lookup
+#   activeDirectory: false
+#   defaultDomain: ''
 #   connection:
 #     host: 'ldap.domain.tld'
 #     port: 389