🚚(api) split API module in client/resource_server

To improve readability and code sharing we group all
APIs into the same "api" module for each application.

Next submodules might be "scim",
"resource_server_scim", ...

The only shared module is the "permissions" one for now.

src/backend/mailbox_manager/api/client/__init__.py

@@ -0,0 +1 @@
+"""Mailbox manager client API module."""

src/backend/mailbox_manager/api/client/serializers.py

@@ -4,7 +4,7 @@ import json
 
 from rest_framework import exceptions, serializers
 
-from core.api.serializers import UserSerializer
+from core.api.client.serializers import UserSerializer
 from core.models import User
 
 from mailbox_manager import enums, models

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

@@ -7,7 +7,8 @@ from rest_framework import exceptions, filters, mixins, viewsets
 from core import models as core_models
 
 from mailbox_manager import enums, models
-from mailbox_manager.api import permissions, serializers
+from mailbox_manager.api import permissions
+from mailbox_manager.api.client import serializers
 
 
 # pylint: disable=too-many-ancestors

src/backend/mailbox_manager/tests/api/mailboxes/test_api_mailboxes_create.py

@@ -8,7 +8,6 @@ from logging import Logger
 from unittest import mock
 
 from django.test.utils import override_settings
-from django.utils.translation import gettext_lazy as _
 
 import pytest
 import responses
@@ -19,7 +18,7 @@ from rest_framework.test import APIClient
 from core import factories as core_factories
 
 from mailbox_manager import enums, factories, models
-from mailbox_manager.api import serializers
+from mailbox_manager.api.client import serializers
 
 pytestmark = pytest.mark.django_db
 

src/backend/mailbox_manager/urls.py

@@ -4,7 +4,7 @@ from django.urls import include, path, re_path
 
 from rest_framework.routers import DefaultRouter
 
-from mailbox_manager.api import viewsets
+from mailbox_manager.api.client import viewsets
 
 maildomain_router = DefaultRouter()
 maildomain_router.register(