✨(oidc) add django-oauth-toolkit w/ configuration

This allows to use `people` as an identity provider using
OIDC and local users.
This commit is partial, because it does not manage a way to
create "local" users and the login page is the admin one, which
can't be used for non staff users or login with email.

src/backend/mailbox_manager/admin.py

@@ -1,6 +1,7 @@
 """Admin classes and registrations for People's mailbox manager app."""
 
 from django.contrib import admin, messages
+from django.contrib.auth.admin import UserAdmin
 from django.utils.html import format_html_join, mark_safe
 from django.utils.translation import gettext_lazy as _
 
@@ -163,7 +164,7 @@ class MailDomainAdmin(admin.ModelAdmin):
 
 
 @admin.register(models.Mailbox)
-class MailboxAdmin(admin.ModelAdmin):
+class MailboxAdmin(UserAdmin):
     """Admin for mailbox model."""
 
     list_display = ("__str__", "domain", "status", "updated_at")
@@ -171,6 +172,29 @@ class MailboxAdmin(admin.ModelAdmin):
     search_fields = ("local_part", "domain__name")
     readonly_fields = ["updated_at", "local_part", "domain"]
 
+    fieldsets = None
+    add_fieldsets = (
+        (
+            None,
+            {
+                "classes": ("wide",),
+                "fields": (
+                    "first_name",
+                    "last_name",
+                    "local_part",
+                    "domain",
+                    "secondary_email",
+                    "status",
+                    "usable_password",
+                    "password1",
+                    "password2",
+                ),
+            },
+        ),
+    )
+    ordering = ("local_part", "domain")
+    filter_horizontal = ()
+
 
 @admin.register(models.MailDomainAccess)
 class MailDomainAccessAdmin(admin.ModelAdmin):