FEATURE: Support for localized themes (#6848)

- Themes can supply translation files in a format like `/locales/{locale}.yml`. These files should be valid YAML, with a single top level key equal to the locale being defined. For now these can only be defined using the `discourse_theme` CLI, importing a `.tar.gz`, or from a GIT repository.

- Fallback is handled on a global level (if the locale is not defined in the theme), as well as on individual keys (if some keys are missing from the selected interface language).

- Administrators can override individual keys on a per-theme basis in the /admin/customize/themes user interface.

- Theme developers should access defined translations using the new theme prefix variables:
  JavaScript: `I18n.t(themePrefix("my_translation_key"))`
  Handlebars: `{{theme-i18n "my_translation_key"}}` or `{{i18n (theme-prefix "my_translation_key")}}`

- To design for backwards compatibility, theme developers can check for the presence of the `themePrefix` variable in JavaScript

- As part of this, the old `{{themeSetting.setting_name}}` syntax is deprecated in favour of `{{theme-setting "setting_name"}}`

app/assets/javascripts/admin/models/theme.js.es6

@@ -188,6 +188,10 @@ const Theme = RestModel.extend({
     const settings = {};
     settings[name] = value;
     return this.save({ settings });
+  },
+
+  saveTranslation(name, value) {
+    return this.save({ translations: { [name]: value } });
   }
 });