♻️(serializers) move dimail calls to serializers

we move all business logic from model to serializer.
all API calls (direct and from front) will keep on triggering
expected 3rd party calls while admin actions will uniquely trigger
modifications in our database.

src/backend/mailbox_manager/models.py

@@ -4,14 +4,13 @@ Declare and configure the models for the People additional application : mailbox
 
 from django.conf import settings
 from django.core import exceptions, validators
-from django.db import models, transaction
+from django.db import models
 from django.utils.text import slugify
 from django.utils.translation import gettext_lazy as _
 
 from core.models import BaseModel
 
 from mailbox_manager.enums import MailDomainRoleChoices, MailDomainStatusChoices
-from mailbox_manager.utils.dimail import DimailAPIClient
 
 
 class MailDomain(BaseModel):
@@ -155,16 +154,12 @@ class Mailbox(BaseModel):
 
     def save(self, *args, **kwargs):
         """
-        Override save function to fire a request on mailbox creation.
         Modification is forbidden for now.
         """
         self.full_clean()
 
         if self._state.adding:
-            with transaction.atomic():
-                client = DimailAPIClient()
-                client.send_mailbox_request(self)
-                return super().save(*args, **kwargs)
+            return super().save(*args, **kwargs)
 
         # Update is not implemented for now
         raise NotImplementedError()