🚚(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