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/discourse-common/lib/helpers.js.es6

@@ -46,19 +46,24 @@ function resolveParams(ctx, options) {
 }
 
 export function registerUnbound(name, fn) {
-  const func = function(property, options) {
-    if (
-      options.types &&
-      (options.types[0] === "ID" || options.types[0] === "PathExpression")
-    ) {
-      property = get(this, property, options);
+  const func = function(...args) {
+    const options = args.pop();
+    const properties = args;
+
+    for (let i = 0; i < properties.length; i++) {
+      if (
+        options.types &&
+        (options.types[i] === "ID" || options.types[i] === "PathExpression")
+      ) {
+        properties[i] = get(this, properties[i], options);
+      }
     }
 
-    return fn.call(this, property, resolveParams(this, options));
+    return fn.call(this, ...properties, resolveParams(this, options));
   };
 
   _helpers[name] = Ember.Helper.extend({
-    compute: (params, args) => fn(params[0], args)
+    compute: (params, args) => fn(...params, args)
   });
   Handlebars.registerHelper(name, func);
 }