The localization translations REST resource sets no cache headers

Note on the "Expected" section: the result also depends on the current user, not only on the wiki, the locale and the keys. Translation documents with the USER scope are registered for their author and apply only to that author's requests. The response must therefore be cached as private. The fix revalidates on every request (Cache-Control: private, no-cache plus an ETag computed from the returned translations), so an edited Translations document shows up immediately.