Merge message_with_kwargs() into message()

message()’s raison d’être as a separate function vanished in a5136074b9:
now that we always show messages in the user interface language (even
the “bulk mode” links after each template on the index page), message()
no longer has a language_code argument that message_with_kwargs()
doesn’t, so we can just have a single message() function that takes
optional kwargs (and otherwise skips formatting).