Cache content cache key on a per-request basis

We do a similar thing with the settings helper, caching settings on a
per-request basis.

Using an instance variable in a helper reduces the amount of times we
need to calculate the cache key during a single request.

Even if Rails caches SQL queries per request, the test suite is faster
with this change, and we get rid of many redundant queries in the logs.

config/initializers/i18n_translation.rb

@@ -8,7 +8,11 @@ module ActionView
 
       def t(key, options = {})
         current_locale = options[:locale].presence || I18n.locale
-        translation = I18nContent.translations_hash(current_locale)[key]
+
+        @i18n_content_translations ||= {}
+        @i18n_content_translations[current_locale] ||= I18nContent.translations_hash(current_locale)
+
+        translation = @i18n_content_translations[current_locale][key]
 
         if translation.present?
           translation % options