💄(domain) enhance admin action label to import mailboxes

So far, "Synchronise from dimail" only import missing mailboxes
from dimail, so this label needs to be more explicit.

src/backend/mailbox_manager/admin.py

@@ -14,7 +14,7 @@ from mailbox_manager.utils.dimail import DimailAPIClient
 # ruff: noqa: S308
 
 
-@admin.action(description=_("Synchronise from dimail"))
+@admin.action(description=_("Import emails from dimail"))
 def sync_mailboxes_from_dimail(modeladmin, request, queryset):  # pylint: disable=unused-argument
     """Admin action to synchronize existing mailboxes from dimail to our database.
     Only works on enabled domains."""