FIX: Couldn't migrate database from nothing due to I18n

Since I18n has a DB backend now, I've introduced a helper we can use to
skip overrides in certain situations. Otherwise migration from empty
databases was broken.

lib/i18n/backend/discourse_i18n.rb

@@ -5,6 +5,10 @@ module I18n
     class DiscourseI18n < I18n::Backend::Simple
       include I18n::Backend::Pluralization
 
+      def initialize
+        @overrides_enabled = true
+      end
+
       def available_locales
         # in case you are wondering this is:
         # Dir.glob( File.join(Rails.root, 'config', 'locales', 'client.*.yml') )
@@ -30,6 +34,15 @@ module I18n
         @overrides[locale]
       end
 
+      # In some environments such as migrations we don't want to use overrides.
+      # Use this to disable them over a block of ruby code
+      def overrides_disabled
+        @overrides_enabled = false
+        yield
+      ensure
+        @overrides_enabled = true
+      end
+
       # force explicit loading
       def load_translations(*filenames)
         unless filenames.empty?
@@ -42,7 +55,7 @@ module I18n
       end
 
       def translate(locale, key, options = {})
-        overrides_for(locale)[key] || super(locale, key, options)
+        (@overrides_enabled && overrides_for(locale)[key]) || super(locale, key, options)
       end
 
       def exists?(locale, key)