Use local_time gem for timezone replacement (#2174)

For the 'time ago' value of stream entry simple statuses and the timestamp value
of detailed statuses - output their UTC value and use the browser timezone to
convert locally.

Gemfile

@@ -35,6 +35,7 @@ gem 'http_accept_language'
 gem 'httplog'
 gem 'kaminari'
 gem 'link_header'
+gem 'local_time'
 gem 'nokogiri'
 gem 'oj'
 gem 'ostatus2', '~> 1.1'