ULS i18n performance fixes

* Since there is an API to load the i18n json files, avoid sending
  list of localizations to client. Previously this list was used to
  avoid 404s for json files. Now API takes care of it. Remove the
  caching for this list too.

* Preload the i18n files on document ready. This avoids delay in
  initializing IME or ULS window after fetch i18n from server.

* ApiULSLocalization can serve messages for all namespaces known if
  namespace parameter is not set

Bug: 50391
Change-Id: I6b848da35f57353790bf460983cdf19a11bb238a