Allowing translations to have default values

[ashleynolan]

Atm, there when we have shared global values (such as in the country selector in the footer) we have to duplicate those values for each set of translations.

It would be better if we could update the i18n handlebars helper to allow us to specify a default set such that:

{
  "default": {
    "countries": [
      {
        "flagUrl": "australiaFlagIconUrl",
        "siteUrl": "https://www.menulog.com.au"
      }
    ]
  }
}

And then if the value for a culture hasn't been set for this reference, it tries to access the default value before failing if no value has been set for either.

//cc @DamianMullins