🚚(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/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(