Refactor to our own Discourse I18n backend

This removes some monkey patches and makes testing easier.
It will also support database backed I18n changes.

lib/i18n/backend/discourse_i18n.rb

@@ -0,0 +1,40 @@
+require 'i18n/backend/pluralization'
+
+module I18n
+  module Backend
+    class DiscourseI18n < I18n::Backend::Simple
+      include I18n::Backend::Pluralization
+
+      def available_locales
+        # in case you are wondering this is:
+        # Dir.glob( File.join(Rails.root, 'config', 'locales', 'client.*.yml') )
+        #    .map {|x| x.split('.')[-2]}.sort
+        LocaleSiteSetting.supported_locales.map(&:to_sym)
+      end
+
+      # force explicit loading
+      def load_translations(*filenames)
+        unless filenames.empty?
+          filenames.flatten.each { |filename| load_file(filename) }
+        end
+      end
+
+      def fallbacks(locale)
+        [locale, SiteSetting.default_locale.to_sym, :en].uniq.compact
+      end
+
+      def exists?(locale, key)
+        fallbacks(locale).each do |fallback|
+          begin
+            return true if super(fallback, key)
+          rescue I18n::InvalidLocale
+            # we do nothing when the locale is invalid, as this is a fallback anyways.
+          end
+        end
+
+        false
+      end
+
+    end
+  end
+end