Guess uid field from first search query

We expect it to be equal or all searches. If not, there is really
nothing that can be done for that LDAP admin.

Related to AlekSIS#249

aleksis/apps/ldap/util/ldap_sync.py

@@ -319,9 +319,9 @@ def mass_ldap_import():
         group_dict = {obj.ldap_dn: obj for obj in group_objects}
 
     # Guess LDAP username field from user filter
-    uid_field = re.search(r"([a-zA-Z]+)=%\(user\)s", backend.settings.USER_SEARCH.filterstr).group(
-        1
-    )
+    uid_field = re.search(
+        r"([a-zA-Z]+)=%\(user\)s", backend.settings.USER_SEARCH.searches[0].filterstr
+    ).group(1)
 
     # Synchronise user data for all found users
     ldap_users = backend.settings.USER_SEARCH.execute(connection, {"user": "*"}, escape=False)