🗃️(contacts) rename base to override

To improve code readability, I propose to rename
the contact field `override`. This comes along
with the fact a contact should not not always
override another (it's the case were I only want
to create some personal contacts).

src/backend/core/api/client/viewsets.py

@@ -152,7 +152,7 @@ class ContactViewSet(
             # - are profile contacts for a user
             user__isnull=True,
             # - are overriden base contacts
-            overriding_contacts__isnull=True,
+            overridden_by__isnull=True,
         )
 
         # Search by case-insensitive and accent-insensitive similarity