FEATURE: Localization fallbacks (client)

This patch sets I18n.defaultLocale in the Discourse.start() script block (it
was formerly always 'en') to SiteSetting.default_locale, and patches
translate() to perform fallback to defaultLocale followed by english.
Additionally, when enable_verbose_localization() is called, no fallbacks will
be performed.

It also memoizes the file loading operations in JsLocaleHelper and strips out
translations from the fallbacks that are also present in a prefered language,
to minimize file size.

app/views/common/_discourse_javascript.html.erb

@@ -33,6 +33,7 @@
   Discourse.Environment = '<%= Rails.env %>';
   Discourse.SiteSettings = PreloadStore.get('siteSettings');
   Discourse.LetterAvatarVersion = '<%= LetterAvatar.version %>';
+  I18n.defaultLocale = '<%= SiteSetting.default_locale %>';
   PreloadStore.get("customEmoji").forEach(function(emoji) {
     Discourse.Dialect.registerEmoji(emoji.name, emoji.url);
   });