Remove i18n symbol dependency

date.order is the only key in rails i18n that is required to be a
symbol. This patch allows for symbols or strings which means:

* No requirement for symbol type in .yml files. A future
  YAML.safe_load wouldn't need to load symbols
* Rails could actually use json rather than yml as the backend
Don't change the original i18n data
Merge pull request #8978 from chrismcg/remove_i18n_symbol_dependency

Remove i18n symbol dependency