FIX: eradicate I18n fallback issues 💣

FIX: client's translation overrides were not working when the current locale was missing a key
FIX: ExtraLocalesController.show was not properly handling multiple translations
FIX: JsLocaleHelper#output_locale was not properly handling multiple translations

FIX: ExtraLocalesController.show's spec which was randomly failing
FIX: JsLocaleHelper#output_locale was muting cached translations hashes

REFACTOR: move 'enableVerboseLocalization' to the 'localization' initializer
REFACTOR: remove unused I18n.js methods (getFallbacks, localize, parseDate, toTime, strftime, toCurrency, toPercentage)
REFACTOR: remove all I18n.pluralizationRules and instead use MessageFormat's pluralization rules

TEST: add tests for localization initializer
TEST: add tests for I18n.js

test/javascripts/lib/i18n-test.js.es6

@@ -0,0 +1,55 @@
+module("lib:i18n", {
+  _locale: I18n.locale,
+  _translations: I18n.translations,
+
+  setup() {
+    I18n.locale = "fr";
+
+    I18n.translations = {
+      "fr": {
+        "js": {
+          "hello": "Bonjour",
+          "topic": {
+            "reply": {
+              "title": "Répondre",
+            }
+          }
+        }
+      },
+      "en": {
+        "js": {
+          "hello": {
+            "world": "Hello World!"
+          },
+          "topic": {
+            "reply": {
+              "help": "begin composing a reply to this topic"
+            }
+          }
+        }
+      }
+    };
+  },
+
+  teardown() {
+    I18n.locale = this._locale;
+    I18n.translations = this._translations;
+  }
+});
+
+test("defaults", function() {
+  equal(I18n.defaultLocale, "en", "it has English as default locale");
+  ok(I18n.pluralizationRules["en"], "it has English pluralizer");
+});
+
+test("translations", function() {
+  equal(I18n.t("topic.reply.title"), "Répondre", "uses locale translations when they exist");
+  equal(I18n.t("topic.reply.help"), "begin composing a reply to this topic", "fallbacks to English translations");
+  equal(I18n.t("hello.world"), "Hello World!", "doesn't break if a key is overriden in a locale");
+});
+
+test("extra translations", function() {
+  I18n.extras = [{ "admin": { "title": "Discourse Admin" }}];
+
+  equal(I18n.t("admin.title"), "Discourse Admin", "it check extra translations when they exists");
+});